I remember asking a similar question at a BitDevs a few years ago as a thought experiment; "How do we get rid of lightning channels? They're the worst." It appeared glib, but I wanted someone to explain why they needed to exist else devise something better.

To understand lightning channels, it helps use metaphors.

Let's imagine you and I:

1. have accounts at the same bank
2. want trade goods and services with each other for money
3. the bank is closed for several weeks

To do this we might record all the transactions between us in spreadsheet, recording what each of us is owed at any point in time, then when the bank opens if I owe you money I send it to you and if you owe me money you send it to me.

If you and I don't know each other well, before we agree to start trading, we might sign a contract:

1. enforcing you have $10 and I have $10 to start
2. enforcing that if I give you $10, I'm not also giving $10 to someone else (because I only have $10 total)

The bank is bitcoin's blockchain and our spreadsheet is a lightning channel. A lightning channel exists to:

1. record the transactions between us
2. prove we own the bitcoin we started the lightning channel with
3. enforce that we don't spend the same bitcoin used in the lightning channel, outside of the channel

Why can a node not just send and receive without a channel?

If you want to pay fees to bitcoin miners every time you transfer money, you don't need a lightning channel!