Why we should download update patch for sims3?

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

Besides, you won't be able to install expansion packs unless you 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 An outdated base game when combined with an updated expansion pack will cause corruptions while playing.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.