In the future, we may plan to add a few items whose proper usage depends on good timing but do not fit in weapon slots, since they are not weapons. Examples include e.g. granting the player a "dash" ability, and arguably even some offensive Domina powers like circle of wrath or shatter. Pressing "I" or "U" then pressing the key for that item can get quite cumbersome, since it requires 2 button presses.
Diablo and many games like it (e.g. Deus Ex), as well as a lot of MMOs, make use of a "hotbar" or "hotkeys" to enable this to work. Those games often feature player abilities that also require good timing and precision to use, but aren't "primary attacks", e.g. offensive spells, mobility abilities, defensive powers. The "hotbar" enables players to bind these abilities to the number keys (as well as a few other keys), and to select mouse buttons, so that they can be used quickly and on-demand.
We may want to consider something like this for Transcendence, but it can be quite complex to implement completely. A much simpler but less functional alternative is to allow selection of such devices as primary or launcher weapons and have them be triggered by the appropriate fire button.