As-built Mechanical and electrical engineering drawings

"as-built" means "as it was built" or, put simply, "the way you built it".

it is the final drawing of a facility/installation, showing all structures, etc, as they were constructed.

the drawing remains valid until there is a modification of the existing structures, after which a fresh "as-built" drawing is prepared.

understand this, and you will figure out any "ten importance" quite easily.

good luck.