This is not the same question as “why become involved in a committed relationship in the first place”; the two concepts are not mutually exclusive.
The older I get, the more I see (with the advantage of hindsight) those close to me jump into marriage essentially unprepared for all its ramifications. In many cases they would have faired much better by simply committing to their partner without getting married, as it is wholly possible to live a full, complete, (be)loved life with another person without a legal certificate.