Why we should download update patch for sims3?

Because the patches are meant to fix the errors players encounter while playing Sims. anda can choose not to if anda like, but then your game will be laggy, while it could have been fixed if anda just clicked that small update button.

Besides, anda won't be able to install expansion packs unless anda update your game. An outdated base game when combined with an updated expansion pack will cause corruptions while playing. Trust me, I know. Bottom line, they need to be of equal versions.