DRAM in a FPGA - Image Sensor design
The RAM has a buffer functionality. This way you can send data bursts instead of trying to synchronize your incoming and outgoing datastream. Depending on the protocol you might have some significant overhead so you want to have as much data as possible packed in a single frame. Using a RAM buffer you can check if data has been stored and then build your frame and the camera doesn't directly rely on a ready USB connection.
So technically you can omit the RAM but I'd at least implement a fifo-buffer (ringbuffer) in the FPGA.