magic is energy. when one casts a spell they are drawing in that energy and focusing it to help them achieve something. magic, however, will not work if after you cast you sit back and wait for change.
if you cast a job spell, you need to pass out a few resume's. magic will make things happen for you sooner than if you didn't cast and sometimes it might never of happened if you didn't.
magic does have limitations, any impossibilities such as flying, invisibility, transformation, won't work. magic is a belief and without 100% believe it won't work for you. i'm a logical person myself, so i can always argue both sides. but at the end of the day, if i desperately need a little something extra, i cast, and it something always happens within the next couple days.
if you don't believe in magic, that's fine, i have a friend who doesn't. it doesn't make you not a wiccan, or whatever you believe, you just don't cast spells. magic is everywhere though, it's a neutral force that doesn't do too much unless we send it to do something.
magic will fail if you don't believe in it, but also if you're casting for fun or to show off, or just because you want to prove yourself. cast a spell that will help you. back to the earlier example, casting a job spell when you've been out of work for months, have passed out hundreds of applications, done callbacks, and have gotten nowhere. you cast with a desperation, a need for the spell to work because there is no other option. this fire is what will make the spell strong and make it work.
hope i explained it to you a bit lol. why don't you want to 'waste your time'? if you're here for some spiritual reason, you don't need to do magic, if you're here to use magic as a way to intimidate others, you can curse/hex people, but it can come back to bite you if that's the only reason why you wish to learn.