The old form is a stone or wooden tablet raised on a stand — an ancestral altar slab, stacked with offerings. That original sense was later written 祖 (ancestor) with the altar radical added, leaving 且 free to be borrowed for its sound as a plain grammatical connector.
Today 且 almost never stands alone; it lives inside 而且 (moreover), the workhorse conjunction of written Chinese, and 并且 (and also). 况且 raises the stakes further: 'what is more'. 暂且 and 姑且 mean 'for the moment', a shrug written into a word.
A stack of offerings on an ancestral tablet — one more, AND one more, AND MOREOVER.
且 was borrowed purely for its sound. The altar it originally drew got the radical 礻added and became 祖, the character for ancestor.