You can update the BIOS if you want, but if your system is running fine then you probably don't have to.
In terms of Spectre and Meltdown, I believe the reason as to why In-Spectre is showing your system as secure, even though you haven't updated the BIOS, is that Microsoft included fixes for these vulnerabilities through updates to W10 and such. So if your system is up to date, then you already have those patches built into Windows. I believe MS did this because they knew that most vendors PC's and hardware makers won't take the time to patch older systems/components, so MS decided to provide fixes for this issues though Windows update to cover all those who won't get a patch through their vendor.
So, I would say that if you have kept your system up to date and In-spectre says your are protected against these vulnerabilities, then I don't see the need to update your BIOS, unless you are having other issues with your system.