@Rainbowgal there are a couple of reasons for asking.
If you have occupational maternity pay and there is a minimum period you have to return to work after maternity leave e.g. NHS is 13 weeks, you can use annual leave for some of those weeks.
The other is about returning to work after Mat leave, you can use annual leave to give you longer at home but also you can use it to phase back into work.
If I assume you work 5 days full time and have 5 weeks annual leave accrued and you accrue another 4 weeks whilst on maternity leave you will have 45 days or 9 weeks leave leave to take. You could return to work 1 day a week and take 4 days leave for 4 weeks (16 days leave), then work 2 days and take 3 days leave for the next 4 weeks ( 12 days leave, 28 days total) then 3 days work 2 days leave for the next 4 weeks ( 8 days leave, 36 days total) etc.
If you are going part time you can stretch your leave even longer as it’s accrued on the basis of you being full time but taken as part time hours.
I do appreciate an extra months pay before you start mat leave is helpful but, as I have often said to women in your position, until you have your baby in your arms you don’t truly know how you will feel. Having the cushion of annual leave can give you more options when you are deciding on returning to work.
Obviously your employer needs to approve a phased return approach but reasonable ones would support this.