The solid (I think that it was the 7 ball) offered Toastie a better opportunity to get the cue ball in line with the break ball - and he had the luxury of using a rail to get there - whereas if he used that 15 in the side, he would have had to draw back out to center. He really could have gone either way with it - but I'll break it down for you.
If you look at how he positioned the cue ball on the last few shots, you will see that he pockets the 15 for an angle on the 2 ball. That angle is deliberate - as it will get his cue ball back to center table. After he pockets the 2 ball, you will see that he has positioned his cue ball in a position where he can go either way ... 15-7 ... or 7-15 ... he has multiple options - it is his choice - and he will choose the route which he feels will be easiest for him.