There's many benefits but one big one that would apply to tech workers is benefits/working conditions guaranteed by a contract where you have a union negotiating those benefits on your behalf.
For example, the vast majority of employers reserve the right to change benefits at any time, and they liberally do in a minor downturn. I've worked at places where an email was sent saying "as of tomorrow we will no longer match your 401(k)." That's it, one executive decided something, you just have to deal with it or quit. Another example is an email saying "this is the new PTO acquiral schedule." I lost a whole week of PTO that day, no apologies, just a "this is how it's going to be." If you're working under a union contract where the benefits are spelled out in the contract, they have to wait until the contract term runs out and then renegotiate another contract to change the benefits.
Some tech companies are known to demand unpaid overtime without prior warning. A union contract may have a provision where you must be paid for overtime or overtime can't be mandatory, etc.
Another example might be that allegations of misconduct must have a transparent investigation before adverse action is taken and the employee is entitled to be represented by the union during the investigation.
If you've only worked in a bull market environment with a massive amount of free flowing VC money you might not appreciate those sorts of things. I've worked outside SV during the Great Recession, during that time jobs dried up and benefits were dropped like a bad date. I was laid off and had to relocate to get a new job. I'm not saying that a union would have prevented that, I'm just illustrating that jobs might not be so plentiful at some point in the future.
Another thing is when a significant portion of a workforce is unionized then there is a tendency to improve the working conditions of non-union shops because they have to compete with unioned shops for employees.
Professional athletes are some of the the most highly specialized, highly paid, and highly demanded employees and the vast majority of them at the highest level are unionized.