When traveling to a new assignment, you have the option to fly to the site and be provided with a rental car, or you can choose to drive your own car. If you decide to drive your own car to the site then agencies will reimburse you with the mileage it took to get there. They will also provide you with the mileage it takes for you to return home (even if you end up just going to another assignment from there). If you choose this route, you will likely make a profit.
For example, if your site is 500 miles away and you were reimbursed $0.545/mile then you will get $270. After deducting $100 that you spent on gas, you will profit $170.
Another mileage benefit that some agencies provide is mileage to and from the site while you are on assignment. That means even though the agency is housing you, they will reimburse you mileage each time you drive to and from the site from your temporary apartment or hotel. Daily mileage is often given when you are doing a local assignment. However, depending on the agency, some will provide this benefit even when you are away on an assignment.
I wouldn’t necessarily expect this from all agencies, but it is a nice benefit when an agency does offer this. It will typically be listed on your contract as general “mileage”. Some agencies may specify your home city and your destination city to display that you will only receive mileage from this commute.
I am used to living about 5-10 minutes away from work so having daily mileage isn’t always essential. Yet, there was an instance where I worked for a rural community health center and the closest hotel was 1 hour away. I wasn’t thrilled about commuting 1 hour each way, but the daily mileage definitely made up for it. I made an extra $300+ each week from mileage alone. After deducting the cost of gas, I profited $200/week. Not bad, especially since it is tax free.