1) Versatility: Nothing feels better than knowing you can play a song or accompany a singer in any key you want. If the singer cannot reach certain notes and wants you to take the song down a whole step, you'll have the confidence and capability to make it happen without hesitation!
2) Revelation: There are certain chord relationships that become evident when you learn every key. Since it is common to find the same chord used in multiple keys, expanding your knowledge of all 12 keys expands your options.
3) Freedom: Musicians who only play in a few keys rely on the "transpose" functionality of their keyboards to survive a church service, gig, or concert. When you become a "human transposer" and can move from key to key without pushing "THE BUTTON," it frees you to play on any instrument, whether acoustic piano or hammond organ!
4) Speed: When you already possess the knowledge of all 12 keys, learning new songs in those once unfamiliar keys becomes drop dead easy. But if you're faced with the task of not only learning a song in "E major" but also familiarizing your fingers with the chords native to that key, you're doing double the work and often times under pressure.
5) Confidence: Ever been in a church that didn't have a musician and you could have stepped up and provided music for the service, if it wasn't for your lack of confidence in playing in all 12 keys? Sitting there thinking if the congregation is singing in a key that you know or not.
6) Adaptability: It is no surprise that certain genres of music and even instruments favor certain keys. Gospel songs seem to have an overwhelming proclivity to "flat" keys. Blues music seems to like F, Bb, C, and Eb. Guitar-driven songs seem to be in E, A, and D. Your knowledge of all 12 keys will allow you to adapt to any surrounding quickly and easily.
7) Respect: Imagine being in a shed or jam session with other musicians and you're impressing them with your mastery of ONE key, and someone utters those dreaded words: "Take that up!" What do you do? Can you take those smooth chords from Ab to A major while everyone's looking? Or do you fold?
8) Fluidity: In the charismatic church, backing up a preacher during the climax of the sermon requires "flowing" with them… especially if they are the type to modulate from key to key. I've seen preachers start out in Ab major and end on Gb in the NEXT octave! Can you follow them on the organ or piano from key to key without breaking the flow?
9) Exposure: If you only play in the black keys, your fingers are used to certain movements and feelings. Certain keys will require you to stretch more than others even though the intervals are the same. For example, in the key of Gb, you can use your thumb to play both Gb and Ab to add spice to a chord. That gives you 4 extra fingers on your right hand to work with. However, in the key of B, can you use that same thumb to play B and C# together? Nope. Learning all 12 keys exposes your fingers to all these little nuances.
10) Leadership: If you aspire to be a music director, you need to understand how to quickly transpose melodies as different instruments in your band will be tuned differently. As a piano player, you may not realize that when trumpet players play "C," it does not come out the same as your C. The same is true with other instruments so if you are demonstrating a melody line for the brass section of your band, make sure you can transpose it in your head before playing it.
11) Stewardship: If you're a gospel musician, there is the responsibility of taking the music gift God has given you and cultivating it. Jesus told the parable of the talents where a ruler left his 3 servants with different amounts of talents (money). When he returned, the one he gave 5 talents brought back 10 talents, a 100% return on investment! The one he gave 2 talents brought back 2! He was very pleased with the both of them. However, the servant he gave one talent didn't bring back any additional talents and instead gave excuses. The ruler kicked him out and gave his talent to the first servant. If the other reasons aren't enough, giving God "back" your best should convince you.
12) Achievement: Mastery of anything feels good. You can fool others with technology and transpose buttons but there is nothing like the feeling of mastering something. Actors get Oscars. Singers get Grammys. Athletes get rings. The common musician may not always get recognition but knowing within yourself that you are versatile, adaptable, and confident pours into other areas of life.