Speaking as a client, I hold over 100 domains that are currently unused. Some I've had for more than 15 years and never pointed them anywhere.
Many of the domains are registered to protect the main domain. Back in the days of running a hosting company, I had variations of our name, misspellings, and also, just to be on the safe side, I also had "HostingCompany-sucks", "HostingCompany-reviews" and variations - mainly for reputation protection as this was a very common thing for competitors to latch on to. Then we had lots of domains point to our main domain too, which were used in marketing campaigns (easier to track sometimes)
I probably have 20+ domains for ideas I want to do, but haven't done them yet. Heck, I'm finally building out a website currently that documents our travels. I registered that 2+ years ago and am only now building a website. I have domains registered and sitting in limbo for ecommerce sites and hobby sites I want to build at some point.
So there are any number of reasons to register a domain. I don't know which is more common, however: registering for a future idea or registering to point to another domain. That'd be an interesting thing to find out.