[wpennington]

It's also important to note that much of the value of any company created in the future as it grows, matures, etc. It sometimes happens that people default to ascribing value based on history or past contributions -- which certainly does matter with respect to the idea, tech, product, early customers, etc -- but is an incomplete ascription of anyone's particular value with respect to the life of a company. Depending on what stage the company is in -- but because we are talking about formation we can assume it is early -- much of the work is likely yet to come. If the past is too heavily weighted, it can cripple the company in the future. Among other things, vesting can help protect founders against over-indexing on the past while ensuring the team is aligned on the future of the company and that are all in it "for the long-haul". Plenty of other reasons to do so as noted in the other comments throughout the thread.

Here is a useful link from Cooley (a large, well-respected law firm) that goes into some of the nuance of these things: https://www.cooleygo.com/founder-basics-founders-stock/