For example, when designing a google docs clone with collaboration, the naïve way to implement that would be to have all the users' computers be sending messages like "insert character 'a' at position 43", etc. The problem with this approach is that it makes the order of events very relevant - you'll get a different result depending on the order you process the messages. The trick is to give each character a unique ID, and then send a messages like "add a character `a` after the character with id 29239810", and then the order is irrelevant.

It happens that not every problem can be expressed in this order-independent way. In general, the ones that can are those that can be expressed in monotonic logic. For some problems, there's no way not to care about the order of events.

If you're unlucky enough to have one of those problems where you have to care about the order of events, you need some degree of coordination between all the nodes in your network to make sure they agree on the order of events. For a high-security project like uniswap, you also want the additional guarantee that the order of old events that the network agrees on is unlikely to change - it's bad if an event that happened yesterday gets reordered or dropped.

Additionally, we're talking about something decentralized here, so we don't want to assume that everyone on the network is honest. If there's a way that a dishonest person can easily stop the network, or cause honest nodes to get out of sync, or cause the order of old events to change, that's really bad.

It turns out that all the good ways to do that look a lot like a blockchain. I'd be eager for you to prove me wrong here.