If you are standing close to a tall building and take a picture of it with your cell phone you would have to tilt your cell phone upward to get the whole picture in.  What happens is that the building looks like it is leaning back.  Look at these pictures:

You can fix the tilting buildings with a program like Photoshop but that leads to other problems. The pictures will have to be cropped losing information on the sides of the picture. The biggest problem with fixing the pictures in Photoshop is loss of quality. Whenever you tilt, bend and stretch a photo, Photoshop has to make new pixels which are based on the nearby pixels. Since Photoshop has to guess at these pixels, quality is lost. Here are the first two pictures fixed in Photoshop:

It’s not perfect and it takes time to do. It’s much quicker and the quality is much better if everything is done correctly in camera using tilt shift lenses.

Now look at these pictures that were taken with a tilt shift lens. They are perfect right out of the camera with the maximum amount of quality.

There may be times when you want the building tilting back for artistic reasons like the last two pictures.  Most of the time though, especially for high end architectural and real estate photography, the verticals should be perfectly straight and tilt shift lenses are the best way to do this.