Enemies drop mods, resources, credits, give exp and missions may contain side objectives.
The game is based around what you do, if you skip doing certain things, you get less overall.

Skiping 100 enemies in a capture mission may sound great, but after 1000 capture missions you will miss 100.000 enemies, it will add up to significant losses comapred with other players.

The more you do in a mission, the less missions you have to run.

As an example, some rare resources only exist in later planets as a drop, meaning crafting your 1st warframe may take you a while, but the same resources may appear in caches early on, so players seeking these caches will be able to craft their 1st warframe sooner.

Effort leads to reward.