Employers generally like to keep track of their employees' whereabouts, both for practical reasons related to things like insurance (as mentioned in the comments) and to prevent abuse in the form of employees using work time for completely personal travel.
Now, in the specific context of your question, it's true that academic faculty enjoy an almost unparalleled freedom to travel whenever and wherever they like during times when they don't have specific duties like teaching. Most faculty are responsible and ethical people who wouldn't abuse this freedom, but in a large department or institution invariably there will be some who aren't. The form that you are describing is a minimal form of oversight that lets the department know when faculty are traveling and for what reasons, and signals to the faculty that they should act responsibly and that ultimately their travel during workdays is subject to scrutiny by their employers. Maybe it is a bit annoying and intrusive, but if you compare this level of scrutiny to what workers in basically all other industries are subject to, I think you will find that academics are incredibly fortunate, and that having to fill a form isn't really a big deal in the grand scheme of things.