Because the jump from 2 naps to 1 is really tiring for them, and it's really easy to end up overtired, and you'll be more likely to get to a reasonable bedtime without OT, from a longer nap!
I'd try it and see what happens if you don't wake her. For comparison, my little one is a lot older, 21 months. She wakes up between 7 and 7.30, goes down for her nap between 12 and 1, naps about 2.5hrs (I wake her up at 3 if she's not up already), and goes to sleep at 7pm. If she wakes up early in the morning, or only has a short nap, I'll put her down at 6.30pm, sometimes after only 3.5hrs A time, and she goes to sleep.