Every competent military commander will always keep a reserve. They do this in case their battle plans go horribly wrong. There have been many times on the battle field where a reserve has won the day. While dating is not quite the same as fighting wars, there are often parallels between the two. Modern daters have learned that it is always wise to keep a reserve for when their current relationship does not work out.
Good relationships are not easy to find or nurture. Partners may see each other for months before they discover they disagree on many of the basics in life. One may want to get married, buy a home and have 6 children. The other person may have the goal of getting married, but they want 1 child and would prefer to rent a place. They would rather move often and experience different lifestyles while their partner would be happier to stay in the same place their entire life. This is where the need for a reserve, such as a fuck buddy is important.
Once it has been established that the partners have different goals, there are only two paths the relationship should take. Either one partner will have to compromise, or they will honestly look at the facts and decide compromise will not solve the issues. This is often a difficult time for two people that have been dating for months. Their relationship is to the point where they are enjoying physical intimacy on a regular basis. Staying together for this reason will not save the relationship. At best, it is a bad compromise and takes away the chance that either partner will find a good match.
At this time, it is best to call in the reserve. The work of fuck buddies is to replace the physically intimate part of the relationship and make a breakup feasible. The ability to find a partner for casual sex makes staying in an impossible relationship a thing of the past.