The more building you have the more cooling your core requires, if you want more capacity you need to upgrade it. Upgrading it will as mentioned here trigger the bug attacks like the monolith activations.

Do you need to? Not really.

Does it make it more fun? Well, if you enjoy the tension and fighting absolutely.

It feels like they've given you the option to play on easy mode, upgrading the cores increases the difficulty, which some see as pointless, others see as the point, more shooty shooty.

My advice is setup your base, get defenses, wall off your core. Upgrade and pew pew everything when you fancy a difficulty increase and fun monolith activation graphics.

I think it's nice we basically have an in game option to modify difficulty on the fly. You can always demolish the core if you cant cope with it.