The concept of choosing a server has really evolved over the years. Way back when MMORPGs first became a thing -- that is to say, people starting playing role-playing games in a large multiplayer setting (before they were called "MMORPGs") -- we didn't have multiple servers. There was one server because there wasn't enough people to justify more.
Then the genre grew and we had games come with with *gasp* 5-10 servers. After the population explosion of WoW, servers became almost ridiculous. There became so many servers it became literally hard to count them all.
Population explosion and opening so many servers has its obvious drawbacks. When populations dip, as they inevitably do, you have some servers become ghost towns and others become overpopulated. You also have a fractured community, and you may have friends on a different server. There are fixes, many of which games like WoW have done. There are alternatives, many of which other games have done.