My cousin is debating moving in with her long-time boyfriend and is sort of freaking out.
She was raised by a very traditional Catholic family (seriously, her dad and brother both went to the seminary to become priests before deciding they wanted a family...) so living under the same roof with your husband before marriage is just not something you do.
See how they live.
This person is going to be your roommate for LIFE. You need to know their living habits, and there are plenty of things that you might not see happen when you aren't around.
Maybe you have a pet peeve you didn't even know about and they drive you INSANE. Better to find that out and work together to overcome it beforehand rather than discover it after marriage, right?
You can test it out.
What if you're worried that after marriage the spark will be gone? Moving in with them might show you how marriage life will be like (since really, after signing a paper your relationship really doesnt change...)
Less stress after marriage.
Getting married is super stressful. Wouldn't it be nicer to, after organizing the wedding and doing all that paperwork, to not have to think about moving all your shit into one house? If you're already living together that's one thing off your check list.
You feel pressured.
What if you aren't planning on getting married any time soon, or, at all? Family, friends, and even your subconscious can make you feel rushed into marriage and that could make your relationship uncomfortable.
Its less of an escape.
Going out to meet your partner, or to have them come over...its a change of scene. When you're living together its so easy to get into a routine and become this old married couple without even getting married yet! Sometimes that little escape from daily life is really necessary in a relationship.