Many people join trade unions for legal protection against claims from people other than (or as well as) their employers.
For example, doctors join the British Medical Association in part to make sure that they'll have the support of highly-paid barristers, paid for by the BMA, if they unjustly get sued for negligence by a disgruntled patient. The same applies to members of the Royal College of Nursing, the Royal College of Midwives and other medical professionals.
Similarly, teachers join the NEU, NASUWT, NAHT or Voice in part because of the support that they'll get in taking legal action against the disgruntled parent who's trying to destroy their career by spreading false rumours about their relationships with pupils.
Pilots join BALPA in part because of the support that they'll get if their career is threatened in some way through the actions of the Civil Aviation Authority or others.
Some people who join unions are even employees, per se. For example, freelance journalists join the NUJ in part to get support when pursuing publishers who fail to pay them the recognised rate for their writings or who simply reproduce freelance work under the name of a staff writer and then make no payment whatsoever.
Further, TTT's argument that a dissatisfied employee can always find another job doesn't hold much water where there are nationally-agreed rates for the work (as there are for nurses and teachers, for example). Unless, of course, he's suggesting that every teacher and nurse who's struggling with their low pay should leave their professions to seek other jobs. If so, there won't be many teachers and nurses left!