For instance, a couple enjoying an otherwise good marriage may have a problem with widely different libidos. One may have a very strong sexual drive, and the other a very weak one. Rather than causing problems by trying to artificially push the weak drive, or curtail the strong one, occasional masturbation on the part of one could solve the problem. For one reason or another, it may be inadvisable for a man to have intercourse with his wife in the latter stages of her pregnancy. Some people must travel and be away from home for long periods of time.
War sometimes separates marriage companions for one, two or more years.
Many women simply cannot achieve orgasm without manual stimulation by their mate. As men grow older, many are overtaken with the problem of impotency. In spite of his problem, the wife could still minister to her husband's need as well. And don't forget the widows and widowers who have no mate to attend to their needs in any fashion. I've pointed out that sexual activity is one of God's special gifts to men and women.