No it's not always necessary. You can simply say, "Mangio una mela" without using "Io". Or perhaps, "Mangi una mela" without using "tu". The verb endings make the listener/reader aware of who it is referring to. However, you would use the pronouns to qualify a sentence when you want to specify who did what. For example, "IO mangio una mela e TU mangi una pera". Hope that helps.

"lo" is a pronoun, that means that if you previously have already expressed the subject, you are going to say "lo". an example: "Mario ha un segreto, ma non lo (il segreto) vuole dire a nessuno"[Mario has a secret, but he doesn't want to tell anyone". Now we have various pronouns that are used with regard to the fact if they are female or male, singular or plural: "oggi quattro bulli hanno preso a calci il professore di religione, ma domani non -lo- (it concerns the entire situation) faranno" [today four bullies kicked the religion teacher, but tomorrow they won't do it again], "Super Mario ha fatto l'insalata con i funghi e l'ha (la ha --> l'ha, the "a" is cut off") conservata in frigo [Super Mario prepared salad with mushrooms and put it into the fridge]. "-Caraaa, i cessi non sono ben sturati-,-non ti proccupare, ci (a loro) penserà l'idraulico-"[-Honeeeey, the toilets haven't been uncorked-,-Don't worry, the plumber will take care of it-],"Il giardiniere ha colto le viole per darle (per dare "esse ") alle prostitutite che ha incontrato ieri" [the gardener caught violets to give them to the bitches he met yesterday] and so on. I will explain the pronouns more in details after you learned a couple of things.