Apparently, a suit is not just a garment it's a beautiful piece of art; hence we ought to frame it carefully and beautifully.
Indeed, when you wearing a suit supposed to be aware of the case that you carry your possessions (laptop, papers, books, files, etc) which ought to compliment your overall look.
I have seen some men wearing nice tailored suits; but unfortunately not aware of the case that they carrying, for instance, the messenger bag, backpack which I would say is a disaster for their look and also harm their suit jacket as well;
significantly the shoulder strap which intensively crushes the shoulder pads, interlining and most importantly the lapels of the jacket, though some men put their mobile phone in their trouser pocket which even looks awful.
However, the case that you carry will enunciate who you are and how you organise yourself; hence I would imply every man should have a nice, well-crafted leather case in their wardrobe.
Although, if you think of choosing the right size; then I should suggest first listing the things that you are supposed to carry for your day-to-day life (try to be minimalist as much as possible) and organise it; now you could find out which size is suitable for you.
And Interms of choosing the colour; preferably match the rest of your leather accessories that you wearing like your belt, wristwatch strap and shoes, but for me, I always match my case with my shoe colour tone which I felt is a nice compliment to my look; despite generally black, brown and tan would be the safest colour tones I perceive.
Therefore, if you are a person who just carries a very few things like a tablet, pen, peppers, or mobile phone then I would suggest you have a well-crafted leather portfolio case (folder) which gives you comfort and a nice compliment to your cool as well.
Well, if you are a person who ought to carry quite heavy things like a laptop, books and peppers, etc, I would suggest you have a nice leather briefcase or satchel which would create a perfect gentleman's look; whereas the advantage of the satchel is it has a detachable shoulder strap which you could. utilise it if your outfit is informal.