Cyclists race by category. Everyone starts as a cat 4. After earning a certain number of points you get promoted to cat 3 and so on. You can only race (and earn points) in races aimed at your category or higher. So a cat 2 rider can’t enter a cat 3-only race. What’s more you have to earn a certain number of points every year to maintain your category otherwise you get relegated (although you only ever get bumped down to cat 3, so even if you haven’t raced in years you will still be a cat 3).
So at the end of every season there is a rush to accumulate as many points as possible, either to desperately hang on to your current cat, or to get promoted into the next one before the season finishes and everyone gets reset to zero. So yes, better to enter an open/men’s race and have a slim chance of points than to not race at all.
As a general rule of thumb female riders are competitive with male riders one category lower. So a female cat 3 rider entering an open cat 3/4 race has a good chance of points, especially if most of the men are cat 4s and she has good enough race skills to follow the right wheels or if the course suits their style - for example a very light female rider may struggle on a flat course but will come into their own on a hilly one. (3s and 4s usually race together in a combined 3/4 race).