At the outset .. default relays cover everything..  so you are covered ..

For fine tuning .. you may want certain relays to do certain things for you .

Relays are hosted by people with specific objectives in mind .. some want to develop new features  (to support new NIPs) .. some want to make money .. in nutshell  ..and a vague anology .. you are choosing different  services from different providers .. 

Also keep in mind ..blasting events to as many relays as possible is not a good startegy .. simply because you are replicating the data at many places .. speed and data .. plus if you want to delete things it gets harder .. 

Getting to a fine balance is kinda hard .. but that discovery is where #nostr fun is ..  once the model gets clear in mind .. we start seeing that social is just a starting usecase  .. nostr is plumbing for a new internet ..