There are a number of metrics that can be derived if you know {something} and time.
Just knowing the task and time is useful from a business perspective for tracking cost of a project. At a business level, you would typically know how much you are paying your engineers, so knowing time allows you to compute the cost of people for the project. Typically, people are the most expensive component of a software project, so knowing this is a huge plus to benefit. It's not the only cost, but it's useful.
From a developer's standpoint, like you said, knowing time can be useful for estimation. The best estimates come from engineers, and to continue to provide good estimates, the engineers need to continually refine their estimates. Some engineers track their time well, while others need a little encouragement. Detailed time tracking is a good way to make (or at least try to make) engineers track their time and improve estimates. However, it's only useful if the engineer wants to continually improve - it's easy to neglect the data.
You can relate time spent on activities to how effective your process is. For example, let's say you are following an agile methodology. In a given iteration, you spend 6 man-hours in code reviews and had 35% of your defects make it through to system testing. In the next iteration, you spent 12 man-hours, and now 30% of your defects made it through to system testing. You spent twice as long on code reviews, yet only detected 5% more defects - this raises a red flag that the code review process needs to be reviewed. You can apply the same techniques to requirements engineering, design, testing, and so forth.
Project effort is also typically measured in time. It's fairly well known that SLOC/time is a poor measure of effort, but measuring some level of output/time (features, story points, earned value) is often used. Sometimes, just the total number of man-hours on the project is tracked. Either way, it lets you know how much time your people are putting into the project.