Purpose of Buffers in this Circuit
It is a buffer and used because the drive source current at hdr P3 is not known. This buffer has almost no load on its inputs but can source +/- 8 mA per pin, even though the package has a wattage limit, depending on if you get a SOIC package or a DIP package. Maximum total current is limited to 50 mA.

That is more than enough to drive the H-bridge drivers. Speed limit for U1 is about 20 MHZ though 50 MHZ is possible with just 15 pF loads. Plenty fast for PWM if you are using that.