Alimony exists because like Tim said, women are underpaid and underemployed. And spousal abuse is still very much a thing: alimony allows an abused housewife an avenue of escape and still be able to manage. They're not on equal footing: she doesn't have the career or job prospects he has, because she's given that up to support his all these years. And even if she were educated the same (or better) than him, she still has to deal with sexism and lower expected earnings.
Without alimony, you allow husbands too much leverage over wives who've been pushed into being financially dependent. Until we've reached full economic gender equality (and there's still a LONG way to go), alimony is absolutely necessary.