They don't in fact you'll be hard pressed to find a modern computer that does. Its a historical remnant. So today it just means screen resolution. (the value has to be something in many formats).
So is it really useless to create an image larger than 72 ppi if the browser will only render it at 72 ppi?
No the value never comes to play. Resolution can mean two things at the same time. It can mean the fidelity of the display device in physical units or it can means total number of imaging elements. PPI value does not change the size of your image file, just the amount of pixels matters, even for print.
See the resolution is your images pixel dimensions. PPI or DPI do not come into play. This info is a piece of metadata where you say I wish this to be displayed as so and so inches wide. However nearly every device will ignore you, in fact even desktop printers can and often do ignore you.
For web and computer imaging it simply does not matter at all, only pixel dimensions matter. Ignore the info (again for historical reasons its considered important, because few decades back all images in Photoshop was destined for print). If you find this confusing then i can tell you your not the only one.