I hate singing in a seated position (even before my gut got rather large), so standing is a natural fix for that. Also, today's venues often have no stage, and I am entertaining on the same level as the audience - standing gives me a visual edge....also more freedom of movement. I have 2 footswitches at my right foot .. one for sustain, and the other for vocal harmonies. Sustain is pushed at least 90% of the night, so a quick shift to the right,and I turn on the "singers".
I put on and take off my guitar a few times each hour, so standing is very natural for me in that regard as well. The biggest reason is visibility though .... the audience can connect with me better and I have more control over the room dynamics when I can look out over the top ... or at least over their heads! lol
Back in the Rhodes days, I sat because I had no way to raise that beast up high enough an d still feel safe, but as digital replaces plywood .... everything just gets easier and easier.
Sit, stand - do what makes you happy, but make the music.