If you want to upload over serial you need a bootloader. That is the usual way to use Arduinos. However, if you use an ISP programmer to upload via Sketch > Upload Using Programmer there is no need for a bootloader and in fact the bootloader will be erased when you do that.The bootloader uses up flash memory. The ATtiny chips have very limited amounts of flash memory so it's best to not waste any on a bootloader.
So, if I dont burn bootloader i only can run ATTiny85 at 1 MHz ?
So the flash memory space will be reduce if run at 8MHz ?