It's very situational.
You can use it to stop a low health enemy from escape, but some champions can jump the wall, or if you poorly aim or time the wall you can stop your allies, and save them. The reverse can be true for using it to rescue a fleeing ally.
Dividing teams during team fights is best when you can cut off escape routes, so ulting straight down lane is not optimal. In this situation it is usually better to aim at even a slight angle to create choke points. If possible, separate front-line from back-line, and if an assassin is near by to delete the back-line, even better.
It is very important to keep as much jungle vision as well. Her Ult is deviating if you can wall someone up in the jungle.
A probably less common strategy would be to simply use her ultimate to rapidly travel between lanes without necessarily walling anyone off. It would be a surprise to your enemy how quickly you left one lane and arrived at another.