what effect does the creature's speech (vocabulary and grammar) have on the reader? why did shelley write it for that purpose?

His babylike speech is innocent and endearing; Shelley wants us to understand that inner part of the creature.

In addition, when the creature explains how he learned to communicate Frankenstein said, “This reading had puzzled me extremely at first, but by degrees I discovered that he uttered many of the same sounds when he read as when he talked. I conjectured, therefore, that he found on the paper signs for speech which he understood, and I ardently longed to comprehend these also; but how was that possible when I did not even understand the sounds for which they stood as signs?” (98).

The Creature also recognizes DeLacey's speech patterns, and then he infers that symbols on the paper represent DeLacey's words. The Creature yearns for knowledge in the same way his creator yearns for the same thing.

The Creature later tells the story about Safie learning French. He says, “Presently I found, by the frequent recurrence of some sound which the stranger repeated after them, that she was endeavouring to learn their language; and the idea instantly occurred to me that I should make use of the same instructions to the same end. The stranger learned about twenty words at the first lesson; most of them, indeed, were those which I had before understood, but I profited by the others” (102).

After this, the creature's mind develops rapidly. He discerns between words and starts to learn new ones. How a newly created monster can learn to speak is amazing. At this point, you can't deny the Creature’s intelligence. Learning a language is difficult for anyone, but the Creature learns faster than the average human. His ability to learn, comprehend, and retain language, and also learn how to read shows how intelligent he is. However, this growing intelligence also makes him aware of the wretchedness of his life.