Because life is like a game of battleship:
As St. Josemaria Escriva puts it:
Watchman, how goes the night? May you acquire the habit of having a day on guard once a week, during which to increase your self-giving and loving vigilance over details, and to pray and mortify yourself a little more.
Realize that the Holy Church is like a great army in battle array. And you within that army, are defending one front on which there are attacks, engagements with the enemy and counter-attacks. Do you see what I mean?
This readiness to grow closer to God will lead you to turn your days, one after the other, into days on guard.
-from the Furrow, 960