ri- a prefix used when something is repeating.
The verbs /tornare/ and /ritornare/ both mean /to come back/.
There are contexts where /tornare/ is not enough and you must either say
/tornare qui/ or /ritornare/.
Example (in a shop, buying a remote control):
a) E se questo telecomando (remote control) non funziona con la mia TV?
b1) Lo ritorni con lo scontrino (receipt) e le daremo indietro i soldi.
b2) Lo torni indietro con lo scontrino e le daremo indietro i soldi.
b3) Lo torni con lo scontrino e le daremo indietro i soldi.
b4) Lo torni di nuovo qui con lo scontrino e le daremo indietro i soldi.
(b2) is ok, although it has too many /indietro/ in it, (b1) is better, (b3) is not
good; (b4) is ok but quite long!
Also there is the word /ritorno/ to mean /the action of coming back/, where
ri- is just 100 % needed.
I would say that we use /ritornato/ when the place of coming back is silent and inferred from the context.
When the place of coming back is expressed, I would use /tornare/.
Anyway if you alway use /ritornare/ you are not making a mistake, maybe some very slight redundancy.
I usually try not to use redundancy in my Italian, unless for emphatic purposes.
the context and