When RE and DE are both low the chip is in receive mode. To send data both pins need to be high then the DI pin also needs to go high. All the 555 circuit does is makes RE and DE high when DI goes high then sets them back to low shortly after DI goes low. So i guess the question is really why is the slight delay needed?

If DI is set to high permenantly then the data pin connects to RE and DE. When data is sent it switches the chip to send mode and because DI is already high it sends a high bit.

Some of my circuits use a 555 and the really small ones use an IO pin to save space. If it could be used how i said then i could save an IO pin or a few components.