This Let_syntax allows you to write expressions like

let open Incr.Let_syntax in
let%map_open some_incr = watch some_variable
and another_incr = ...
and ...
in
...expression involving some_incr, another_incr, etc...

Note that this is less efficient than using map3, map4, etc., as the latter produces fewer intermediate nodes. You can also use let%mapn syntax to use n-ary map functions efficiently.