Discuss and learn French: French vocabulary, French grammar, French culture etc.
I don't think you need to use them for capital letters.
Otherwise I think you do need to know and use them.
Yes, optional on capital letters (I've often wondered why) but e and é are actually considered different letters. They're on different keys on an AZERTY keyboard.
Yes, you need to use them (though some will say it's false, they're commonly absent from capital letters, as George and stu both stated). They often come from an "historical" "s": écouter coming from escouter.
It gives an indication on how to pronounce the word, and it helps distinguish between words that would otherwise be the same: mur/mûr = wall/ripe, or mémé/même = granny/same.
Better yet: côte/côté/coté/cote = rib (or coast, this one already has two meanings!)/side/rated/value (in stock market)