Why was this case worth going to court for let alone all the way to the supreme court?
It shouldn't. There's already ample precedent at both the state and federal level for such things -- grant money can be awarded so long as it's not used for "specifically religious purposes." Thus, for example, my church has received both state and federal grants for structural repairs because it's a historic building that happens to be a church. On the other hand, we cannot apply for state/federal grants to repair our stained glass windows, because they have an explicitly religious purpose/message. So we're funding those differently.
For this preschool, if they a) don't limit the preschool to members of that church; and b) the preschool playground does not serve a specifically religious purpose, it could in fact be eligible for the state grants.
The value of taking a case like this to the USSC is that it would act to moderate the overreach on "separation of church and state."