Some of the best things come in small packages.
Take his miniature magnifying glass. It's compact, practical, travel-size – he can take it anywhere, and does. It serves it purpose without being obtrusive. It fits in his fingers perfectly, and quite often those fingers will stray to its familiar shape whether he needs it or not. It's a reminder to him. It's his tool. Just as necessary as the coat-gloves-scarf trio.
Additionally, take John, for example.
John, who is just as unassuming, a little compact, mostly practical, and is always travel ready whether he wants to be or not. Like his mini-glass, John follows where Sherlock leads. He even helps with cases, providing actual verbal insight, rather than amplifying clues that form verbal certitude.
Both are important.
Sherlock is getting close, not quite there yet, but he is getting close. Because, like they do for his glass, his fingers twitch more and more towards John. He suspects soon that John will take the most precedence, will be more vital to Sherlock than his little extension of plastic and glass.
One day, yes, very soon, John will be more familiar to his touch, his fingers, will be a weight he knows more intimately than anything ever before.
Until then, he reaches once more into his pocket, and ponders the actual non-smallness of both.