While not suitable for absolutely everyone over my years of coaching I have found that those who are able to swim consecutive lengths at a pace of around or quicker than 2:00min/100 do benefit from learning to tumble turn. Why?
1) Speed. First and foremost when performed adequately it is quicker and I'm yet to met a triathlete who doesn't want to swim quicker. As a non-scientific experiment I have timed athletes who are adequate at both touch and tumble turns and found that generally tumble turning is about 1 second per length quicker. This equates to 15 seconds over 400m. How much training would it take to be able to swim 15s seconds quicker over 400m? Spending a bit of time learning to tumble turn can bring huge time benefits.
2) Breathing control. Touch turning allows an extra breath to be taken at the end of the length, for some this is an opportunity to pause and gain composure for others it can be an excuse for a break, whichever the reason, the extra breath provides your body with more oxygen - a good thing yes? Maybe not. Learning how to reduce the number of times you breath and putting yourself in situations where your breathing is challenged can be good training for open water swimming when you turn to breath and are faced by a wave or a neoprene clad body meaning you are not able to get the breath you need. Learning to control and at times reduce your breathing frequency is an important skill that will transfer across to open water.
3) Fitness. The reduction in breathing through a turn and eventually, through practice increasing the number of strokes that you take out of the turn will gradually help increase your fitness and therefore increase your ability to swim quicker before you get out of breath.
4) Fluidity and rhythm. Think about how you turn when touch turning. In many cases I see, it involves reaching for the wall, taking hold of the wall, lifting your body slightly as you twist round to re-position your body to allow your head to face the direction you want to go, it is often slow and sightly cumbersome even when well performed. When practiced and competent a tumble turn allows you to swim in a straight line into the wall rotate smoothly and push off the wall in a straight line. This gives the stroke greater fluidity and rhythm, vital components of high performance.
5) Confidence and mindset. For some tumble turning makes them feel like a swimmer, feeling like a swimmer in the pool can improve your comfort and confidence to be there. Confidence is paramount to being able to perform at your best. If speed is success for you and you swim quicker in training (see point 1) then you are more likely to see you are successful and therefore approach your training in a positive/confident manner. More success in training means more confidence stepping up to the race and increasing the likelihood of performing well in a race. Perform well in a race and your confidence improves; it's an upward spiral!
So go on take the first step (or somersault!)