Use a TSE lens to correct the converging vertical lines in architectural photography or when doing packshots.
Another often used feature is working with selective focus in your pictures. The 90mm lens is good for this when shooting closeups of jewlery or watches.
Also portraits or makeup shots can get a special feel when the focus is just on the eyes or on the lips.
The other way around you can use the TS-E features to put the focus even more in the layer of your object to increasing depth of field f.e shooting food on a table with your camera placed on a tripod in front so you have a 90° difference between sensor level and your objects.
To say it short: a TS-E lens convertes your DSLR into a small largeformat camera giving you not all but most optical effects you would otherwise only get with bellows camera system.