Solvents get you high. They also kill you, but beggars can't be choosers in the post-apocalypse.
People inhale spray paint in real life if they are desperate enough, or just really stupid.
From 2008: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2442441/
> Sniffing the fumes of aerosol spray paint or "chroming" is the most popular form of inhalant abuse within Australia
Probably not a coincidence given that George Miller is Australian.
Also consider how many people live now and their resources and assets, compared to the world of mad Max, where perhaps 1% of the humans remain. It implies a lot of junk.
Weird cult practices are very human and all over the place, from the outside they don't make sense. Especially in warfare, strange psychotic over the top practices are exemplified many times. "This is a practical and reasonable thing to do" doesn't really come into it. Although, to be fair, getting your warriors in an unhinged altered state of mind is a quite effective way to motivate them to do what you want.