I think we can all relate to this. Be you cartoonist, artist, illustrator or graphic designer. At one time or another and most likely even now, we have all had that single beloved tool. Be it a Staedtler pigment liner 0.1, a Faber Castell PITT artist pen M or perhaps a Derwent Graphic pencil 3B. It could be Round Sable brush #7, or perhaps its the Dr Ph. Martin's brown india ink you use it in. It's more than likely that at one time you've entrusted your working ability to a single implement.
During it's life that tool can become essential. It can become the make or break element to your work. Maybe yours is a £30 Rapidograph rotring pen, it could be the Argos pencil you accidently left the shop with. Regardless of what it is you use, or how you use it, you become familiar with it. You become familiar with the way it reacts to certain stock. The pressure it requires to give that perfect tone. How best to handle it along side different media. You perfect it's use to the stage where it may come to feel like an extension of your ability.
(I'm dedicating this post to my beloved mapping pen)