There is some overlap here with ChrisW's answer. The point I would like to emphasize is that for both monks and lay persons, there are benefits of admirable friendship.
Admirable friendship is important for both monks and lay persons, so that they may be positively influenced by the wholesome and skillful qualities of their admirable companions.
With admirable friendship, a lay person could progress on the right path according to AN 8.54:
[The Blessed One said:] "There are these four qualities, TigerPaw,
that lead to a lay person's happiness and well-being in this life.
Which four? Being consummate in initiative, being consummate in
vigilance, admirable friendship, and maintaining one's livelihood in
tune.
"And what is meant by admirable friendship? There is the case where a
lay person, in whatever town or village he may dwell, spends time with
householders or householders' sons, young or old, who are advanced in
virtue. He talks with them, engages them in discussions. He emulates
consummate conviction in those who are consummate in conviction,
consummate virtue in those who are consummate in virtue, consummate
generosity in those who are consummate in generosity, and consummate
discernment in those who are consummate in discernment. This is called
admirable friendship.
Meanwhile, SN 45.2 speaks of the importance of admirable friendship for a monk:
"And how does a monk who has admirable people as friends, companions,
& comrades, develop & pursue the noble eightfold path? There is the
case where a monk develops right view dependent on seclusion,
dependent on dispassion, dependent on cessation, resulting in
relinquishment. He develops right resolve... right speech... right
action... right livelihood... right effort... right mindfulness...
right concentration dependent on seclusion, dependent on dispassion,
dependent on cessation, resulting in relinquishment. This is how a
monk who has admirable people as friends, companions, & colleagues,
develops & pursues the noble eightfold path.
Sutta Nipata 1.3 below states that it's better to wander alone as a monk, if one does not have admirable companions. Of course, having admirable companions is the best option.
If you gain a mature companion,
a fellow traveler,
right-living & wise,
overcoming all dangers go with him,
gratified, mindful.
If you don't gain a mature companion,
a fellow traveler,
right-living & wise,
wander alone
like a king renouncing his kingdom,
like the elephant in the Matanga wilds,
his herd.
We praise companionship — yes!
Those on a par, or better,
should be chosen as friends.
If they're not to be found,
living faultlessly,
wander alone like a rhinoceros.