Note 3 at Rom. 16:2: The archaic meaning of the word "for" was, "because; since" (American Heritage Dictionary). Paul is saying that the reason they should assist Phebe is because she had assisted others, including Paul. This illustrates the law of reaping what you sow (Gal. 6:7, Lk. 6:38).

Some people become offended when they do not receive assistance from others, yet they have never helped anyone. That is not what Paul is advocating here. Phebe had earned their help. Salvation is by grace, but respect and help from others has to be earned.