why do they sometimes use quiero [i want] and sometimes yo quiero [i want]

In English when you say "want"....there is no way to determine if YOU want, THEY want or WE want...and then you add an "s" to the end and you get "wants", an of course the only options for "wants" is either HE or SHE or IT....maybe im still not being clear right now , but ill get there!!! dont worry... In Spanish we have a "want" for each person...by person i mean ( he , she , it, we, us, they, blah blah). In the case of "QUIERO"...its only used for I or "yo"...and you get "YO QUIERO", but if you dont wish to use the "yo" cause it comes implied, you can simply used.."QUIERO"...using the same "quiero" word, when you say "HE WANTS" the word "quiero" changes to "quiere" and you get "EL QUIERE"...if you remove the "EL" the sentence still makes sense...but , we can say its better if you use the "EL". In the case of "they"...it would be "ELLOS QUIEREN"...notice the "n" at the end...and for "us"...NOSOTROS QUEREMOS....so...if you go to a bar with a couple a friends and the waitress asks "WHAT DO YOU WANT"...you can simply say "(Nosotros)QUEREMOS 3 CERVEZAS"....or you can add the "NOSOTROS" but again this comes implied....cause its not correct to say "YO QUEREMOS... :S it doesnt make sense at all!! mucha suerte!!! nos vemos pronto.... have a nice day... and excuse my english ;)