Cats like to be up high. The RSPCA says cats often feel safest when they are high, and recommends giving every cat a raised spot to climb and rest. A cat tree is one way to do that. It is not the only way (sturdy shelves and the top of a wardrobe work too), but for many homes it is the easiest.
The best cat tree is the one that does not wobble, has perches big enough to lie on, and sits where your cat already wants to be.
The RSPCA lists "high places" among what every cat needs, along with a bed, a toilet, hiding places and room to climb and play. A raised spot lets a cat watch the room, rest out of reach and get away from things that worry it. Cats Protection also advises plenty of cat-friendly shelves and things to climb on for indoor cats, so they can explore as they would outdoors.
In a home with more than one cat, the same sources say each cat needs its own things, with a spare, and places to be away from each other. A tree with only one good perch can become something to guard rather than something to share.
Welfare groups do not set a standard for cat trees, so these are common buying-guide rules of thumb that fit what we know about cats:
Put the tree where the family already is, ideally by a window or in the living room. A tree in a spare room is usually ignored. Keep it away from radiators and hot windows in summer, and do not put it right beside a door a cat can use to slip out. If your cat is old or stiff and has stopped using high places, mention it to your vet, because pain can be the reason.