There are many, many things you can do yourself to create your ideal home landscape: put in a lawn, add a raised bed, install a deck (maybe!), etc. On the other hand, unless you have personal experience in outdoor construction, there are some projects that really do need professional expertise: main stairs, retaining walls, front walks, in-ground pools, etc.
Any project where visitor safety is a factor (uneven or slippery stairs or walks can cause serious injury) or where redoing a botched project would be costly (poorly installed retaining walls tend to lean and collapse, a badly seated in-ground pool can not only leak, but could rip out half your lot if it collapses suddenly) deserves proper installation by professional landscapers who know what they’re doing.
And while you’re at it, make sure that the people you hire have excellent references and offer a solid guarantee!