A well-balanced party composition should have some kind of support class, which the Cleric happens to excel at. The best Cleric builds aren't necessarily those that deal the most damage ...

The Knowledge Domain Cleric gets a revision in the latest Unearthed Arcana playtest, proving once again mind really matters.

Clerics are a classic option for healers in D&D, but there are a lot of other classes that can hold their own as healers.