If you still find the subject in which your took your degree is absolutely facinating and you want to spend the rest of your life doing that, then you stay in that subject.
Often, four to six years studying in one narrow field shows you that it is not something you really want to be doing all day, every day, for the next forty years or so.
But what you have learnt, besides the details of your subject, is that you are capable of focussing your attention on something and getting very familiar with it, and you have learnt how to organise your time and abilities to get the most out of the material you can study.
The degree proves to a potential employer that you have leanrt these things, and he may employ you to do a job which needs those qualities but not in the direction you were previously going.
My daughter studied biology to honours and PhD level, then did a yearlong postgraduate course in "Computing for Biology" which was mostly on using computing in analysing research data, but gave her a grounding in computing for general use too. After her second graduation, she was offered five different positions in different firms, and chose to go to IBM - nothing biological within miles. She now works for the General Registers of Scotland, on applications connected with digitising all kinds of historic Scottish records for the internet. Not what she studied to do, but a worthwhile and rewarding job.
Not everyone knows at 18 exactly where they will want to be at 30, 40 or 50.