Hate to say it; material goods or vacations.
Money from the day job goes to paying the bills / mortgage / living expenses. Money from side work goes to toys be it a new laptop, phone, vacation, remodeling a room of the house, etc...
I probably wouldn't freelance if I was content with what I have and soon, that freelance money will start to be saved for a down payment on a bigger house instead of going towards the latest iPhone.
2ndary to this is the drive to keep abreast of new technologies. Sometimes, I can do freelance projects in a different language or framework - which makes me marketable for further side work. IE: I hated Wordpress - but then I started getting requests to do freelance work in Wordpress, so I learned it. Knowing Wordpress has scored me more work then when I ust' to write everything manually in PHP. (Not that Wordpress is new; but I wouldn't have learned it unless I was requested to write in it, for example)