It means if you're scared enough by the intentional vagueness of the term, either somehow contribute to fulfill the clause or don't use the software. Perhaps that's not how it would be interpreted, IDK, I'm not a lawyer.
You may get contribution-spam fixing inexistent errors, just to fulfill the clause.
If developer X is working in company Y and a year later X changes to company Z, who can use the software Y or Z? What happens if X dies? What happens if Y goes bankrupt and W buys all the assets?
At least it leaves some leverage in the hands of the authors/contributors against flat out freeloading. Normal MIT is essentially no strings attached.
The intent of the clause is to encourage both, if you contribute, you can do whatever but maybe it doesn't work that way.