The overall idea behind this is that BGP is meant to be a Border protocol literally, and your AS is supposed to be like your Home turf. eBGP is more like outsiders. BGP does not want to get too hung up on the intricacies of routing WITHIN your AS, and expects you to have that stuff managed by an IGP. So Think of your AS as your House. If your mom yells Dinner is ready, you don't go knock on your brother and sisters doors (iBGP neighbors) to let them know she said that, you are assuming that they have their own rapport with mom. If Guests (eBGP neighbors) are over, on the other hand, your spouse says Dinner is ready, you make a point to let each individual guest know that because they are not IN the 'circle of trust'. This is a very silly analogy but it really works in my head to understand why that happens.

This is the Split-Horizon rule for iBGP. The overall idea behind this is that BGP is meant to be a Border protocol literally, and your AS is supposed to be like your Home turf. eBGP is more like outsiders. BGP does not want to get too hung up on the intricacies of routing WITHIN your AS, and expects you to have that stuff managed by an IGP. So Think of your AS as your House. If your mom yells Dinner is ready, you don't go knock on your brother and sisters doors (iBGP neighbors) to let them know she said that, you are assuming that they have their own rapport with mom. If Guests (eBGP neighbors) are over, on the other hand, your spouse says Dinner is ready, you make a point to let each individual guest know that because they are not IN the 'circle of trust'. This is a very silly analogy but it really works in my head to understand why that happens.