Let's compare for example
(defn lookup-users-i []
  (query (get-db-conn)
         '[:find [?user ...] :in $ :where [?user :user/name _]]))

to
(defn lookup-users-ii [db-conn]
  (query db-conn
         '[:find [?user ...] :in $ :where [?user :user/name _]]))
The first version is easier to invoke via the REPL because you offload any db connection setup logic to the get-db-conn function. You don't need to worry about building a connection and passing it in. On the flip-side, at the lookup-user-i call-sites you don't have arguments going in, which provides folks reading the code with less context regarding the function's behavior.

One could make two different arities of the function:

(defn lookup-users-iii
  ([] (lookup-users-iii (get-db-conn)))
  ([db-conn]
   (query db-conn
          '[:find [?user ...] :in $ :where [?user :user/name _]])))

This way, one gets the best of both worlds, at the cost of a little extra typing upfront.