The theory behind probation (and parole, although that's rare in Virginia anymore) is that it is agreement between the government and the defendant. The government forgoes a certain amount of punishment and the defendant agrees to forswear all his evil ways. Of course, the carrot comes with an explicit stick: "Screw this up and we'll throw you back in." Typically, a defendant will receive a sentence of something like "5 years with 4 years 10 months suspended, the suspended time to remain suspended for a period of 5 years with 2 years of supervised probation." At any time within the 5 years that the time is suspended the court could impose some or all of it for failing to abide by conditions the court has imposed (new convictions, failing to remain on good behavior, etc.). Realistically, while the suspended time could be imposed at any time, it is unlikely to be imposed unless the defendant is still on probation to have someone to report him to the court.
Defendants almost never concern themselves with the amount of time or conditions which attach to probation. They have myopic vision which zeros in on how much time they are actually getting for their conviction, ignoring all else. Despite their attorneys' advice to the contrary, I'm fairly certain that the vast majority of felony defendants would agree to 10 years of probation rather than spend a month in jail and a year of probation.
The purpose of probation is fairly straight forward. Of the four purported reasons for a judicial criminal sentence - rehabilitation, quarantine (keeping the defendant from further harming the community), deterring others from offending, and simple punishment - probation falls almost exclusively in the rehabilitation. It's society's attempt to put someone back on the street and keep them on the straight and narrow.
The reality is somewhat different. Pretty much everyone in the courtroom, except perhaps the defendant himself, expects to see many (probably most) of the defendants back in the courtroom during their probation. This is based upon common experience repeated over and over and over again. Certainly, not all people put on probation violate it (and God bless those who don't), but the experiential bias which comes from being in the courtroom every day wouldn't lead judges, defense attorneys, or prosecutors to bet on it.
In some courthouses this can lead to a "we'll get him later" attitude which leads to sentences that are low because prosecutors and judges think they'll have another go at this guy a year or so down the line when the offender is back before the court in a probation violation hearing. In that hearing the level of proof is lower and the rules of evidence are far easier (basically, the hearing proceeds with all sorts of hearsay reported by the probation officer). It is extremely rare that a defendant is found not to have violated his probation and most probation hearings are primarily about what the punishment for the violation will be. You'll even hear defense attorneys trying to take advantage of this attitude in plea negotiations: "C'mon Mary, this is John Smith. You can give him a short sentence. You know you'll get it on the backside."
The flaw in probation is that offenders and society at large don't associate punishment meted out under the probation system with the original offense. They see it as punishment for the probation violation. Thus, incarceration which proceeds from a probation violation only serves the purpose of quarantining of the defendant. The fact that punishment is occurring means that rehabilitation has failed. Community deterrence doesn't occur because the punishment is divorced from the crime (although, assuming the offender hangs around with others on probation it will at least serve as a lesson to them). Simple punishment for the initial crime went by the wayside in the initial sentencing; the punishment in a probation violation hearing is for the failure of the offender to reform himself. Still, the offender ends up incarcerated.
Herein lies the philosophical fork in the road. If someone believes that there is a class of persons who are going to break the law no matter what is done, then the goal is to get these individuals off the streets for the longest period of time possible and the "We'll get him later" method make the most sense. It allows offenders to be removed from society in a far easier manner than a full blown felony trial with its constitutional protections and high level of proof. It also provides a safety valve because those very few who have the fortitude to fly straight and actually make it through probation without violating don't get an unnecessary period of incarceration (a benefit to both them and the taxpayer). On the other hand, if someone believes that individuals, and through them society as a whole, can be taught and learn through their errors then delaying punishment for the initial crime and putting people in prison later for probation violations which they don't associate with their original offense is counter productive. If the first felony larceny offense only gets someone a 3 year suspended sentence and a year of probation (a fairly typical sentence), no one learns not to commit larceny. At best, they learn to live by the rules of probation. This does not shape society away from those acts which we have deemed serious enough to make criminal.