Looking for words with a given letter can help you better understand how common and frequent that letter is in the English language. For instance, everyone knows that vowels are very common in English, but it is interesting to see how they do not show up in equal measure in the dictionary. Likewise, you may also find out that certain structures repeat themselves countless times even if you never noticed before.
Searching words that contain a specific letter is also a good activity for Scrabble and Words with Friends players. It can help them perceive and understand why each letter is valued differently in these games, for instance.
Grasping how common a certain letter is can also be useful when planning which words to study or to use in these games. Players can then analyze the word probabilities for each letter and focus on those with the less frequent but high-value.