Confusion about `IndexOffset`

[RocketRide9]

Hello. https://lukaswirth.dev/tlborm/decl-macros/macros-practical.html#indexing-and-shuffling: Here book introduces IndexOffset struct. I don't understand why it's needed.

This

                        let next_val = {
                            let $ind = MEM_SIZE;
                            let $seq = &self.mem;
                            $recur
                        };

instead of

                        let next_val = {
                            let $ind = self.pos;
                            let $seq = IndexOffset { slice: &self.mem, offset: $ind };
                            $recur
                        };

seems to work fine.