To play games from a sata hard drive. Why else? I am sure you are aware the Dreamcast didn;t ship with a HDD, so modders came up with many ways to play games off of different storage media, and this happens to be 1 way of a few now that you can play games.
Obviously but still, why? Why ruin a perfectly good Dreamcasts GD Rom drive for a stupid HDD mod? If anything, buy an external SD loader.
I hope you realize this actually prolongs the systems life by taking a lot of the work off of the disc dive which is very notorious for failing on the Dreamcast. I mean, these things are coming up on 20 years old now. Don't you think it is better to add a HDD or GDEmu to the machine instead of letting them die? I had my main DC disc drive die last year, so I can attest first hand how shitty the disc drives are. Also, I am sorry but no way no how will you ever be able to play every single game from the slow serial port lol. Seems to me like you aren't that well versed in the Dreamcast homebrew community so I will forgive your ignorance and calling the hdd mod dumb.
Here is a tutorial a friend of mine wrote up for adding a HDD to a Dreamcast. https://fnhack.wordpress.com/
If you don't have a GDROM drive in the machine any more, you need to replace the boot ROM with a modified one that contains the DreamShell bootstrap. I assume that's the flash ROM that's in the photo in the OP.
EVERYBODY take a moment to go and read the site rules. Your behaviour is appalling. You're adults - stop acting like children in the playground at school. @UnrealNStuff we don't tolerate abusing other users. Quit swearing at people. @Anthony817 that does NOT excuse you reciprocating. Be the bigger person - don't stoop to his level and throw insults back. ANYONE carrying on will be having a time out from the site.
for boot DreamShell from HDD without disc need BIOS mod if you have GD-ROM need flash boot_loader_devkit.bios or boot_loader_retail.bios if you don't have GD-ROM need flash boot_loader_devkit_nogdrom.bios or boot_loader_retail_nogdrom.bios
Ok nice so I need to mod the bios to boot the hdd without the gd rom. The gdrom is there but don't read disc anymore and I don't want to look for lens to replace it.