You don't have to do anything SupportAssist nags you about, especially not BIOS updates. If you don't want to be nagged, you can disable or uninstall SupportAssist.

Dell frequently installs components that are somewhat different from their retail versions, eg video cards. So they may tweak drivers for Dell-specific hardware. 

Windows Update may install drivers that work, but they may not be optimal for Dell's OEM hardware.

Windows Update doesn't provide BIOS updates, so those have to be gotten from Dell's Support page for your specific PC model. Rule of thumb is to install BIOS updates only if they fix a problem you currently have. Installing BIOS updates to fix security issues is worth considering vs the risk hackers may exploit the hole to attack a PC.

Do not let SupportAssist manage BIOS updates. Download the file manually to the desktop and read/follow the instructions carefully to run the update. A failed BIOS update can permanently destroy a motherboard, and Dell has policies about when they will or won't replace a motherboard after a failed BIOS update.