Wherefore then
The answer is sixfold:
(1) The law was added because of transgressions, that is, to give to sin the character of transgression.
(a) Men had been sinning before Moses, but in the absence of law their sins were not put to their account. (Romans 5:13). The law gave to sin the character of "transgression," that is, of personal guilt.
(b) Also, since men not only continued to transgress after the law was given, but were provoked to transgress by the very law that forbade it; (Romans 7:8); the law conclusively proved the inveterate sinfulness of man's nature. (Romans 7:11-13).
(2) The law, therefore, "concluded all under sin." (Romans 3:19); (Romans 3:20); (Romans 3:23).
(3) The law was an ad interim dealing, "till the seed should come". (Galatians 3:19).
(4) The law shut sinful man up to faith as the only avenue of escape. (Galatians 3:23).
(5) The law was to the Jews what the pedagogue was in a Greek household, a ruler of children in their minority, and it had this character "unto" that is, until Christ. (Galatians 3:24).
(6) Christ having come, the believer is no longer under the pedagogue. (Galatians 3:25).
because of
For the sake, that is, in order that sin might be made manifest as transgression.
See (Romans 4:15); (Romans 5:20); (Romans 7:7); (Romans 7:13).