Well the tangible benefits to women (and my definition includes transwomen of course) are that ALL women (including transwomen) are represented within feminism. It would be a bad idea to start excluding groups of women because they are different to other women - it needs to be a united movement to work.
To "work" in what way, for what purpose?
I think before we should say who Feminism is for, we need to understand what the purpose of Feminism is.
To me, it's to counterbalance and eventually dismantle the historic sexism that has lead to women and men having unbalanced powers and roles in our society to the disadvantage of women.
In most cases, that means stopping something - for example VAWG, deliberate sexism, unconscious bias, expectations of caring and domestic labour placed unequally on females in the family, generally socialisation and social roles.
In some cases, it means treating female and male people differently because of our different physical capabilities and reproductive roles - for example maternity leave, sports.
In many cases, it means temporarily treating female and male people differently until we can get the underlying problem stopped - for example most physical single sex provision is there as a refuge from VAWG, female-only shortlists are there to counteract unconscious bias.
Therefore in my view, the group of people whose needs are addressed by Feminism is not defined by the specific word Woman and therefore by whatever the word Women happens to mean today, but as the group of people who suffered that historic sexism. So while I'm fine with the word Woman including trans women going forward, that does not mean the sexism suffered by female people retrospectively applies to trans women as well so nor should it mean that the rights and protections put in place because of that sexism are automatically transferred to trans women along with the label. Whether you believe that sex should be a meaningful disntinction between humans or not, the simple fact that for millenia it was used to separate humans into woman and men and the women-by-sex did and still do suffer sexism because of it makes it meaningful, at least in the context of who Feminism is for.
Can Feminism fight for trans women? Absolutely. In the areas trans women's needs are aligned with female people, such as challenging gender stereotypes or male violence, Feminism's work to dismantle historic sexism will benefit trans people as well. Should Feminism include all needs of trans women in its definition simply because they are now labelled as Women, even when those needs are not aligned to the overall goal of counterbalancing and eventually dismantling the historic sexism that has lead to women and men having unbalanced powers and roles in our society to the disadvantage of women? No. Because Feminism isn't a free-floating cause that can be tagged onto any group that has the right label, it was shaped for and by the women - the female people - who needed it, and if you disconnect it from those people what you have might be called Feminism but it's no longer the same thing.