Usually it is the longer security and other updates period if you are lucky and your device has a LineageOS maintainer. E.g. Nexus 6 is from december 2014 but still supported. But if you have both up-to-date stock Android and LineageOS, then the only benefit I see having LineageOS with gapps, is that you have a little more control. You can flash pico gapps and just add only these gapps components you really need...
No gapps here since the beginning