When to use float vs double Java?

The answer to your question - When to use float vs double Java is -

LibGDX is a framework mostly used for game development.

In game development you usually have to do a whole lot of number crunching in real-time and any performance you can get matters. That's why game developers usually use float whenever float precision is good enough.

The size of the FPU registers in the CPU is not the only thing you need to consider in this case. In fact most of the heavy number crunching in game development is done by the GPU, and GPUs are usually optimized for floats, not doubles.

And then there is also:

memory bus bandwidth (how fast you can shovel data between RAM, CPU and GPU)

CPU cache (which makes the previous less necessary)

RAM

VRAM

which are all precious resources of which you get twice as much when you use 32bit float instead of 64bit double.