Good question. The independent labels were regarded (rightly or wrongly) as indicative of authenticity. If it was on a major label it was treated with suspicion. But most importantly, if you were browsing and came across a new release on a label you knew had released stuff you'd bought and liked, you might be more likely to risk your 70p. For me. Step Forward, Rough Trade and Beggars Banquet were promising (as was Sire, an American lable), but Stiff had a lot of recycled pub rockers (among gems like Elvis Costello) so a bit more caution was in order. 70P was a lot of money in 77.
Apologies if the past below is too big for your phone. I have blocked each year into alphabetic order. I think that if you squint you can see punk transition into 'New Wave' or (my preferred terms) 'alternative pop music' over time. I haven't listed all the soul music, early 60s trash pop. or any of my massive 12" singles collection. Something for another time. 
