Especially if you've been playing this game for so long that it became too easy. It makes the campaign much more interesting and in most instances a bit more challenging imo. Not expanding like crazy, sometimes completely stopping, but trying to optimize the economy/military infrastructures as much as possible at the same time requires a bit more finesse and it ultimately leads to a more interesting game : the IA gets time to grow in power but so are you, meaning that you'll have more interesting battles to play, and with top tier units. It adds a bit more depth to the map campaign, making demography and switching castle to cities or the reverse much more important. It's also a very roleplay friendly way to make godlike generals, developing their traits is much more easier if you have a stable predictable front. It's also fun to boost your reputation and see what you can do with diplomacy even if it's messy (especially on VH). There are others indirect reasons to turtle, like making the hordes much stronger/scary if they knock on your door but by that time you might be strong enough to beat them. Of course i'm not advocating for a game where you sit on your butt and do nothing, the end goal of this is to still win the game, but win it with style. Atm I'm playing as England in the Britannia campaign and i actually managed to make this big red blob interesting thanks to the turtling philosophy.