Strike is about withdrawing labour. Labour is sometimes the only bargaining power some workers have. Their general election vote counts, but not for trying to change their workplace. By banding together and striking for a common cause, pressure can be put on employers to come to the negotiating table and start really listening. Pay is the most common reason for striking but poor working conditions, changes in contract being pushed through to the detriment of the people working in them, reductions in pensions, general bad relations, people being sacked because they are members of the union, wanting to be paid the same as someone else doing the same job as you, bullying, sexism are reasons that have been cited in the past.
Sadly sometimes the public are caught in the cross-fire but the people still active in unions have seen what the exploitation of people not in unions and don't want it to happen to them.
It's not like in the 70s when there were strikes every 5 minutes over things like arguing over which union had first dibs on the people in the factory etc
People don't strike now unless they have a very good reason. It's been made impossible to strike for anything less. I would personally like a higher turnout in the ballots but there you go. It's still not perfect and people sometimes don't know what they've got until it's gone.
Teaching is getting walloped (sp???) and the teachers have no other way to fight back. (I'm not a teacher. I am a union member and I support my sisters in the other unions.)