Trust and money.
Serious tensions in the past have resulted in serious misconceptions about the other side's capabilities and intentions, a relevant example with nukes would be the Cuban Missile Crisis. For a few years prior to the crisis there had been a lot of rhetoric in the US about a 'bomber gap' and a 'missile gap' that existed between the US and the USSR, with the US way behind the USSR in the inventories of both. It was known to the US government and the USSR that was a load of bunk but it was certainly a factor in rising tensions between the two. Knowing they were far behind the US the USSR took a gamble and put missile in Cuba, that failed but not before almost sparking a war.
A large part of the point of the subsequent treaties we made with the USSR and Russia was to increase both sides knowledge of their respective inventories, intentions and general capabilities. The ultimate result was increased level of trust on both sides that we weren't planning some sort of surprise attack on each other or had some other nefarious nuclear related plans. The only way these things work though is that both sides follow the treaties, which surprisingly enough with respect to the US and Russia that has generally been the case. The ultimate result of that trust is reduced tensions and a lessened chance of nuclear conflict between the signatories.
Another big reason is money. Like anything else nukes and their delivery systems cost money to design, build, test and deploy. A treaty cutting the amount of weapons saves both sides money, which we could both use.
Ultimately this is a pretty good win-win for both sides. The previous START was closely followed by both sides and gave us great insight into Russia's nuclear arsenal, and vice versa, and it allows us to reduce the amount of arms (money) that we maintain in our arsenal but keeping plenty to still blow the world up if we want to.
Good enough?