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 ..
 

 In essence, nostr is a market place for data services .. you can choose from hundreds of relays ..and 100s of clients ..to suit your needs  .. hardware etc .. even start your own ..

Once you curate an experience for you , you can provide it to others .. build your own tools around it .. its like a chess board .. simple in the beginning , becomes more and more complex as the game progresses   :-)  

 Relays are data services. 🤔
I like this framing. 
nostr:nevent1qqs9ruehfeqq77fyvwqe2sx07dehwjslp508lf9lm8paj6qal9ttjqcpz4mhxue69uhhyetvv9ujuerpd46hxtnfduhsygz67pu5vp4ptdtyrcj65g7sft6vkrtate5tz89vk3l9dfrf3l9gcspsgqqqqqqs625ukz