The primary value is that a union is the only realistic way a worker in the US can get their employment terms specified in a contract. Without a contract, these terms can in many cases be changed arbitrarily and with any amount of notice.
For example, Yahoo made remote employees either relocate to an office or resign in 2013 [0]. With a contract, the process by which this change could be made could be defined in advance.
Severance is another example. In many companies, the amount of severance an employee will receive is not disclosed in advance. A common process is to offer the lowest amount the company thinks the employee will accept and still sign a separation agreement. This penalizes employees without substantial savings. And since there's a short window to exercise potentially expensive stock options after termination, many employees have an urgent need for a large lump sum of cash right away. The employee is in a weaker position without a contract specifying severance.
A contract could also do things like limit non-compete clauses. These are already non-enforceable in California, but are still common in other areas of the country.
Judging by the disclosures in the proxy statements of public tech companies, executives frequently have these types of provisions in their employment contracts. Because of their position, they're able to negotiate these individually; regular workers need to act collectively to get similar benefits.
[0] https://money.cnn.com/2013/02/25/technology/yahoo-work-from-...