Why do you presume that all people from the same place are automatically friends or even like each other? When you have people with histories there's camaraderie, but there are also rivalries, stolen girlfriends, family disputes, revenges, all kinds of histories and skeletons in the closet that can cause problems later on. On the other hand, starting on a clean slate avoids this - and in my own experience at least, spending 3+ months in a bootcamp 24/7, eating, training and sleeping with the same guys creates by itself quite a strong bonds between people and a sense of brotherhood (united against the officers mostly, but still the brotherhood).

In the case of the pals battalions, the point was that the UK's tiny professional army had been shattered in the battles that opened the war, so they needed to drum up a whole new army in a very short period of time. But unlike the continental powers, Britain had no tradition of mass conscription; British armies had always been made up of volunteers, and the government knew backing away from that policy would cause a political uproar. So some way had to be found to encourage lots of new volunteers to come forward quickly.

The pals battalions were the government's attempt to solve this problem. They offered potential recruits a kind of carrot: groups of men could join them together, with the guarantee that the group would be kept together in the same unit. You and your mates could go down to the recruiting office and sign up to support each other for the duration. Going to war is a scary thing, so that was an appealing proposition.

An unspoken stick accompanied the offer, of course. It was understood that if not enough volunteers could be found a draft would probably come eventually, and if you were drafted, you'd just get thrown into whatever unit the government chose to toss you into. So if you were a man of military age you were probably going to end up in the ranks one way or the other -- but if you volunteered, you could at least do your service among friendly faces.

The combined carrot and stick worked beyond the government's wildest expectations, drumming up a huge New Army (see https://en.wikipedia.org/wiki/Kitchener%27s_Army) in just a few months. But then that New Army got fed into the meatgrinder of the Somme, and the plan that had seemed so brilliant revealed its dark side as communities across Britain woke up to the realization that none of their men would be coming home.

In the last century there were also practical concerns like making sure soldiers could understand each others' accents.