Why is it that I can accept some assignments directly, while others I have to apply for?

There are two types of assignments on our platform: Projects and Bookings.
Projects are usually less extensive and expect a clearly defined result from you, e.g. 'Write a blog post with 500 words on the topic of freelancing'.
You can accept projects that match your skills with just a single click and start working on them immediately. Any communication with your client takes place–if it's even necessary–exclusively via the chat feature.
Bookings, on the other hand, are often much more extensive and open-ended. The client is usually looking for someone to work on complex or long-term projects and will probably want to get to know you beforehand–maybe through chat, video call, or email. The application for a booking remains non-binding until you both make a decision to work together. Of course, you can always withdraw an application for a booking at any time, and you don't need to provide a reason.