There are at least two major reasons why feminists should work towards a politics of coalition. One of these is pragmatic and the other is more ethical.
On the pragmatic side, feminists need to have political help. Feminists are not numerous enough to allow them to make up a political majority all on their own. Therefore, they need to pursue a strategy of coalition building to give them the power that they need to implement any sort of important changes.
Ethically speaking, it is also important for feminists to engage in coalition building. Feminists are interested in fighting against the ways in which the dominant society treats women in an inequitable way. They should also, therefore, be opposed to other forms of inequitable treatment for powerless groups in society. They should be opposed to things like sexism or poor treatment of senior citizens or people with disabilities.
For these reasons, it is important for feminists to work towards a politics of coalition.