To what end do miners typically put "nonstandard" transactions

[Kimbrough Software]

I'm reading Franco's book "Understanding Bitcoin" and greatly enjoying it.   Franco says these are usually written by miners on bespoke node code bases.  What are the main purposes of such transactions? I have discovered that, separately, transactions with dates in the future (non final) also get dropped from the mempool.  As too do double spends.  But i'm curious to know how people are using these non-standard txs.