You also need to keep track of it for rations and resting. Basically, you as the DM have to be aware of how far a PC can actually go before he just needs to sit down and rest for a while, possibly even get to a point where they're so far along they might need to camp in the dungeon.

And, of course, the all important wandering monster check.

Plus, presumably, they've got obligations outside that are pressing on their time, too.

It's largely the same reasons you track time in non-old school games.