Because your baby is working against gravity in this position, the organization noted that it could be helpful in maintaining a good flow of milk. (romper.com)
Parenting suggests working against gravity as your baby eats (and for a few minutes after a feeding) to see if that helps before assuming your baby needs medical attention for it. (romper.com)
My let down was too fast, and I read somewhere that feeding while lying back works against gravity, and may help. (powerfulmothering.com)