Why does an author use a subplot in a novel

Which excerpt from chapter 3 of the strange case of dr. jekyll and mr. hyde illustrates a character vs. character conflict? hosts loved to detain the dry lawyer, when the light-hearted and loose-tongued had already their foot on the threshold; they liked to sit a while in his unobtrusive company. the large handsome face of dr. jekyll grew pale to the very lips, and there came a blackness about his eyes. “i do not care to hear more,” said he. “this is a matter i thought we had agreed to drop.” “my good utterson,” said the doctor, “this is very good of you, this is downright good of you, and i cannot find words to you in. i believe you fully; i would trust you before any man alive, ay, before myself.” but i do sincerely take a great, a very great interest in that young man; and if i am taken away, utterson, i wish you to promise me that you will bear with him and get his rights for him.”