Manual focus is great in a lot of situations. AF has some intrinsic limitations given how it tends to work. One of its biggest detractors is that it will usually only autofocus around a general point, or possibly a locus of points. This is not a huge deal when precise focus is not important, such as wide hyperfocal landscape shots. In other cases, such as macro shots, portrait shots, or anything where exact placement of focus is important, autofocus often fails, or does not perform adequately.
Another case where manual focus can be a saving grace is when autofocus has a problem consistently focusing in the same place. It may pick the right thing to focus on most of the time, but every so often it may decide to focus on something else entirely. This can be mitigated by selecting different AF pints or loci, however even that may not be sufficient at times. If you ever run into a situation where your autofocus usually focuses on the right thing, but every so often hunts and focuses on something else, you might find it useful to use AF to focus on the proper target, then switch to manual to prevent further hunting. This can often occur at night or indoors where lighting may be low or inconsistent.