They can be used for almost anything... Since they support USB natively in addition to Thunderbolt extras like HDMI and DisplayPort as well as PCIe lane access to power extremely fast storage or other options.
You may see TB devices start dropping in price over the next year or two as USB4 supports TB3 (which is backwards compatible) and should lead to a wider adoption rate at a lower cost.
I know they can be used for displays, but is there anything else they are good for? I've seen pcie enclosures, but they cost hundreds of dollars, unfortunately, and egpus aren't officially supported for Th2 anyway.
all the things you can do with a TB3 port, can be done with a TB2 port using the Apple TB3->TB2 adapter.