If you have a problem you just have no idea how to deal with, you might go in for short term therapy. Maybe for a time or two. Therapists should be able to help you sort yourself out pretty quickly.
If you have a specific problem, such as a fixation on porn, or an inability to go to work, or are having trouble with a spouse, that would be good to see a therapist for.
There are also longer term, more serious issues. Others have mentioned them: mental illnesses of all kinds would appear here. Depression, bipolar, schizophrenia, eating disorders, ADD and all kinds of other things are in this category. It’s where you have a diagnosis from a Psychiatrist, or you have a chronic problem that is bothering you. These kinds of conditions are a slam dunk as far as deciding that therapy is needed.