what is difference between client.add and client.update?

[skim0011]

https://docs.mem0.ai/platform/quickstart#4-5-memory-history

client.add changes its info well, why do we need client.update?

example:
user alex info {name=alex, vegetarian=true}
message = "I recently tried chicken and I loved it. I'm thinking of trying more non-vegetarian dishes.." client.add(memory_id, message, user_id="alex")
user alex info {name=alex, vegetarian=false}