Belle and Sebastian are a completionist’s nightmare. Much of their output has been scattered over singles (a good chunk of which were handily rounded up on 2005’s excellently titled Push Barman to Open Old Wounds), and nearly all of it has been of remarkably high quality. Released two years into the band’s hiatus, The BBC Sessions should be inconsequential. The album collects songs recorded at the BBC between 1997 and 2001, as well as a 2001 concert in Belfast — coming from any other band, it would scream “stopgap.” Yet The BBC Sessions is an essential part of the band’s discography.

A big part of that is the Belfast bonus disc. Though they started off as more of a studio project, Belle and Sebastian have evolved into a phenomenal live act, and 2001 marks the beginning of the band’s conscious effort to focus on improving their performances. The Belfast recording finds the band stretching out in front of a highly appreciative crowd. They’re relaxed, taking requests and playing covers as the audience claps along. Two of the covers are unsurprising — the band nails the dreamy “Here Comes the Sun,” and anyone who’s heard “I’m a Cuckoo” from Dear Catastrophe Waitress knows the band can handle Thin Lizzy’s “The Boys are Back in Town.” But when they bring a random fan onstage for a take on the Velvet Underground’s “Waiting for the Man,” the hard groove immediately dispels any notion that Belle and Sebastian are too twee to rock out.

The actual BBC sessions, on the other hand, document the end of an era for the band. The four tracks that finish off the disc mark the last contributions of cellist Isobel Campbell, before she left to pursue a solo career. They also never made it onto any of the band’s official releases, and were previously available only through bootlegs. As for the songs that have appeared elsewhere, the versions of tunes like “Lazy Line Painter Jane” (here simply “Lazy Jane”) and “Sleep Around the Clock” are every bit as captivating as the studio versions. It’s an excellent primer on the first half of the band’s career, a near-perfect introduction to the band for newcomers and a frustratingly necessary addition to the band’s catalogue for completists.