If it takes 5 minutes to read from RAM the same read will take 7 months to read from a spinning disk. So saying “disks are slower” sometimes doesn’t communicate how much slower they actually are.
If your index doesn’t fit in memory the slow down you will experience when querying on that index as pages are read from disk expired and then reread from disk will be dramatic.
The goal for every performant database is to have all its regularily queried indexes to fit completely in RAM. To futher increase performance the most regularily queried records should fit in RAM as well.
For a gaming application this means data associated with logged in users, for a banking application accounts that have transactions in the last thirty days etc. etc.