You generally want to make sure you're working out specific muscles or muscle groups a certain of times each week and with a certain number of working sets, rather than "I have used $MACHINE recently so I need to do that today."

I only have dumbbells, so selecting the equipment first is actually ideal for me

Ah yes that makes sense! I would think of that as more of a pre-workout filter? Where you select the equipment you have available and it automatically filters out everything that requires equipment you don't have access to.

Yeah, that's what the feature is for, not "what equipment do I want to use".

Because the purpose of exercising is usually not to use a certain machine but to train a certain muscle (group).

The feature is definitely not meant to serve as option for what you want to use, but what you can use. You could use it that way, of course, but that's not what it's there for.