As you cast the spell, you draw a 10-foot-diameter circle on the ground inscribed with sigils that link your location to a permanent teleportation circle of your choice, whose sigil sequence you know and that is on the same plane of existence as you. A shimmering portal opens within the circle you drew and remains open until the end of your next turn.
You can create a permanent teleportation circle by casting this spell in the same location every day for one year. You need not use the circle to teleport when you cast the spell in this way.
The teleportation circle you draw seems to be a point of departure only when you cast it. When it's a permanent teleportation circle, however, it doesn't mention whether you can use it as a point of departure.
Can permanent teleportation circles only serve as destinations?
Permanent teleportation circles are for creating "custom" teleport destinations, such as a party's home base. Permanent teleportation circles themselves are only destinations, as you said. They don't allow you to teleport from them without casting the Teleportation Circle spell.
I disagree with some of the thoughts here. Let's start with the sigils. The sigils in your temporary teleportation circle connect your casting location (think coordinates) with sigils that represent the location where the permanent gate is. Based on this, there can only be one location chosen per usage. This means that if made permanent, there is only that one destination permanent circle per permanent circle.
Once made permanent, you don't have to cast again. That's the point of being permanent. It's always open one way.
So how do we get back? You need one on each end, except you can't build them simultaneously aimed at each other based on the rules. You actually would need two permanent circles on one end and one permanent circle on the other for back and forth.
Your first permanent gate can only connect to one you are already are familiar with and know the sigils (address) for. So you can't connect it to your other proposed gate, because it's not built yet. So your first permanent gate connects to someone elses. Once you are finished the first gate, you have your "home" gate with complete sigil address. You then get to the location you actually wanted to connect to by teleport/walking, or whatever. At that location, you make another permanent gate (your 2nd), that will exit to your first gate at home. So now you can go from your destination back to home. Once home, you create your second permanent gate there, to connect to your 2nd gate, the away destination you just got the sigils for. So you leave home by traveling from your gate 3 to gate 2. And return home from Gate 2 to gate 1. We would assume gates 1 and 3 are close by. Or maybe the return gate is in a secured area, so nothing can follow you home.