Erik Darling runs through an exercise:
The more columns you have in a table, the more potential column combinations there are for indexes. Much like columns, indexes tend to get added following the path of least resistance.
Very rarely does someone consider current indexes when deciding to add an index.
Erik’s process is a good one. The real pain comes when there are 40-50 indexes on a table (seriously…) and there are a lot of similar-but-not-quite-similar-enough indexes.