(Labour were elected unopposed last time in this ward.)
As I wrote before:
A loyal core vote comes in part from people getting the habit of repeatedly voting for the Liberal Democrats – and that requires repeatedly giving them the chance to do so.
Not having a candidate means we’re refusing to let voters be loyal repeat supporters of us and instead forcing them to turn elsewhere – to another party or to apathy. That’s not the way to build up in weak, middling or strong areas.
By-elections are also a great opportunity to give new people the chance to learn how to do things, such as being an agent for the very first time. If your local party is so over-flowing with experienced agents that you don’t need any more this doesn’t matter much… but for the rest of us in places where willing agents are often in short supply and over-worked, by-elections in seats we’re not campaigning hard to win give a great opportunity for someone new to learn a little bit more about what’s involved.
The trick, of course, is not to say “we’re too busy to do this” but instead to say “it’s just because we’re so busy we should use this chance to help build a bigger team”.
Note: there are of course some very rare occasions when not standing a candidate is understandable in a small local party. My points are general ones which apply nearly all the time, but not quite in every single case over the years.