The agreement will show in detail who owns what and contribution during the relationship eg what happens if one party stops work and has a child. The agreement can also address how you will share your property, your personal belongings, your savings, and any other assets if you were to separate and sell the house/move out.
It can be used to agree how you will manage your day-to-day finances during your relationship, for example how much each of you will contribute to paying the rent or mortgage, and your bills.
It can even detail whether you intend to take out life insurance and arrangements as to pension benefits as an example.