One, because s/he prefers to read cogent and well-edited chapters, rather than having to wade through chunks and pages of confused thoughts and tiresome verbiage.

Two, s/he has your interests at heart (getting your dissertation completed, so you can get out — on time!)BEFORE your funding runs out… your spouse threatens to divorce you, or… your teen says he never wants to go to college, “because my dad started his PhD way before I was born, and I don’t think he’s ever going to get done”)… all true stories, by the way.

Three, in all likelihood, your adviser has worked with me before, or has seen how I salvaged a doctoral candidate from the depths of academic despair, holding him or her accountable, one chapter at a time, and knows I can help you before you slide too far into the Academic Abyss of No Return.