Where it's an advantage to have friends in the mountains? (4,6)

'friends in the mountains?' is the definition.
The definition suggests a plural noun which matches the answer.

'where it's an advantage' is the wordplay.
I cannot quite understand how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is located in the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is present in the answer.
This explanation may well be incorrect...

'to have' is the link.

Can you help me to learn more?