Some days it's too wet to get outside on real rock and some days climbers just can't make it to the crag. Some are lucky enough to live near craggy hotspots but even then it can be hard to fit work, family life, and climbing into a single day or weekend. The indoor climbing gym is always an option but if you've got sturdy doorframes you can also hang fingerboards and bring the gym into the house.
They're particularly good for climbers who want to excel on rock with small, crimpy holds. In a conventional gym and even an indoor climbing gym, it can be hard to build up the finger strength you need. Careful, long-term hangboard training means bigger routes on smaller holds and fewer injuries along the way. If you've got the space, you can build a home climbing wall or 'woodie' with a bag of professional quality, bolt-on holds. If you really haven't, try a set of Metolius Rock Rings. They can be hung almost anywhere and there is no need to drill holes in the wall to mount them.