[coryrc]

Dams literally generate energy and provide water consistently.

[selectodude]

And lay waste to entire river ecosystems.

[obmelvin]

Do dams provide water? Don't they use the water provided by a body of water?

[dredmorbius]

By controlling and regulating water flow, dams make available more water than would be available from an unregulated water flow.

As noted by others, this isn't entirely cost-free, and there are ecological impacts. Still, on net, many dams do provide real benefits.