There are two main ways to look a Sroads: as extra VP for late game engines and as a rush card like Gardens.
The later is the easier to wrap your your head around. A typical Garden rush averages maybe 2 cards gained per turn and piles out 26 cards (Gardens, Estates, and an enabler like Iw). This makes your typical garden worth ~3.6 VP and the game ends at T13 at the earliest.
Sroad can do the same magic with a few cards and in the process they can bulk up to 21 VP cards making each Sroad worth 5.2 VP.
This makes uncontested Sroad more point lucrative, but both top out shy of of a province pile and both suffer from the fact that an engine player will gain a higher VP delta from taking the $4 green than provinces. Both are still doable with the right setups, but you need something pretty good once your opponent nabs 2 or 3 of your main green.
Outside of the rush and the rare green heavy kingdom, Sroad is mostly competing with Duchy. Some things are obviously better with Sroads - e.g. Followers without estate trashing will give you a huge boost towards Sroad as does Tournament generally. At 12 green Sroad are strictly superior to Duchies outside of obvious edge cases. A typical end game win for an engine is to hit 3 Prov, 4 Duchies, 1 Estate. That is painfully close if you both kept estates. If you expect to lose the province pile, then you are extremely likely to want Sroad over Duchy. If you lose Provs badly (e.g. 6:2) or Colonies are in the mix, then you will absolutely want to go Sroad over Duchy.
If you are mixing other alt-VP in, then the "beats duchy" threshold kicks in much sooner. Beating provinces requires you to carry 24 VP cards, but can be doable with setups like Iw/Mill or Nobles/Smason.
Sroads is extremely powerful for mass gain scenarios, like with Troll, Smason, Dev or Possession as you may be able to pop an entire VP pile or two in a single go. Piling out both the Sroad and Estate piles generates 40 VP with 16 gains, not easy but something that can allow you to win with a slow starting engine.