A lot of good reasons already listed, I'll add mine though they are probably already covered.
I have two nearly identical Durst L184 Dichro enlargers. The reasons behind getting the second (I already had a Beseler 4x5 as a second which was replaced) were as follows:
1) it's a pain getting a negative in the glass carriers perfectly clean and free of Newtons, so once I have that set up right, I'd rather not remove it until I'm done. The problem is that sometimes you get stuck on a print, or need to walk away from a certain image/print for a day or so in order to get a fresh perspective. In the past that meant that I either had to remove the neg or not print at all for a few days. With 2 matched enlargers I can just move on and print on the second one.
2) backup, getting parts and repairs for many enlargers is not quick or easy now a days. Having a second enlarger means that I can continue to work, and meet deadlines even if one enlarger craps out on me.
3) Diptychs are far easier to do and match if you can do them side by side
4) while one test print is drying down or washing, I can start on a second negative.
5) BY getting a "matched" second enlarger all mixing boxes, lens boards, neg carriers, light bulbs, etc are interchageable between the two. Exposure times are the same and the "feel" of working the enlarger is the same because all the controls are in the same places.
6) in the event of a tornado my house now has far greater ballast