[LegionMammal978]

Yet some parts of math are 'preferred' over others, in that most 'serious' mathematicians would rather read 100 pages about functional analysis than 100 pages of meandering definitions from some rando trying to solve the Collatz conjecture.

Some people would like to have a filter for what to spend their time on, better than "your elders before you have deemed these ideas deeply important". One such filter is "Can these ideas tell us nontrivial things about other areas of math?" That is, "Do they have applications?"

Short of the strawman of immediate economic value, I don't think it's wrong to view a subject with light skepticism if it seemingly ventures off into its own ivory tower without relating back to anything else. A few well-designed examples can defuse this skepticism.