Valued = useful + hard to find.

The more rare you are, the more you are valued. You still have to be useful to be valuable though.

In order to acquire those rare skills, an ungodly amount of work is often required. It is also a huge variable. A rare skill today can become useless tomorrow.

Also, beware of indispensable people. "This company needs me" can have all sorts of causes. Maybe you didn't documented things right, maybe you just have a long history with them. It doesn't mean being rare. Being rare is feeling "this skill I have could be of use to any similar company and few people know it".

The skill could be anything. Perfect pitch, excelent document writing, de-escalation, low-level bit shaving.