Can using legacy mode instead of UEFI mode affect my laptop in any way?
It won't cause any damage.
Legacy mode (a.k.a. BIOS mode, CSM boot) matters only when the operating system boots. Once it boots, it doesn't matter anymore. If everything works as expected and you're happy with it, legacy mode is fine.
Advantages of UEFI boot include:
- Faster boot times. UEFI can skip initialization of some devices which would be reinitialized by OS anyway
- Optional extra security. You can enable Secure Boot, which checks digital signatures of OS components, ensuring that they weren't tampered with. You should combine it with BIOS password protection.
- If you're using multiple OSes, UEFI offers boot manager integrated into firmware.
- If you're using Windows, UEFI mode lets you use GPT partitioning scheme, which supports disks over 3 TB. (Linux can use GPT without UEFI)
Is it bad? Not at all. Switching to legacy mode might confuse or break a bootloader that expects UEFI, but you can always switch back if you aren't able to boot. Nothing will be permanently changed.
Original BIOSes were quite simple. They initialized hardware, set up drivers that could be called while in real mode, copied the first sector from the primary storage device, the MBR, to address 0x7C00 and executed it. The actual BIOS itself remained in the first page of memory where it held objects that were needed if returning to real mode.
Modern UEFI systems are significantly more complex and do a lot more under the hood. However, some operating systems expect BIOS behavior and won't work with UEFI. The solution is CSM mode, or legacy mode, where a UEFI module is loaded that exposes itself to the operating system like a genuine BIOS.
The operating system doesn't need to be aware of UEFI-specific behavior in CSM mode. See OSDev's page on UEFI for more information.
I have used both types thousands of times literally! As a senior tech for law enforcement, public and private institutions of higher learning, and private and public schools. I can tell you its a matter of the age of your machine. Some machines fully support it (newer), and some barely support it (several years old), and some older machines never had it or should have never had it! It offers speedier boot times and some added security for your machines, if you have survived this long on a legacy machine stick with it, it wont hurt. If you have important data that can cause great harm to you or your institution if your machine is stolen then I would switch to UEFI mode and implement the added security it can provide. A few of the others have described some of the things that can be done.