[nobody9999]

>What's the big benefit of fusion that makes it worth investing so much money?

4-20 times the energy density of fissionable materials[0] and fuel that's the most common element in the universe.

[0] https://en.wikipedia.org/wiki/Energy_density#In_nuclear_reac...

[d_theorist]

And what is the benefit of that?

[nobody9999]

>And what is the benefit of that?

I'll leave that as an (fairly obvious) exercise for you, dear reader.