in terms of usage, if I say "people should buy what they need and on top of that they should not spend (lest they find themselves spent)" you know exactly what i mean. Especially expensive items are spendy. Spend carries a connotation of waste and exhaustion. Save and invest your surplus, don't spend it, don't spend your savings.
Squander means to spend thoughtlessly or foolishly or wastefully. It's not a synonym of "spend".