Is there a difference between clicking “reply” and writing a new tweet with @somesone?

Question

On Twitter web, when I click the reply button for a tweet, there's @someone inserted in front of the message automatically.

Is there any difference compared to writing a new Tweet and entering @someone manually?

Answer 1

There is a difference. When you click "reply", your Tweet will be associated to this other Tweet and shown as "in reply to…" it.

If you simply add @username to a Tweet, this won't be linked to another tweet—only the user.

From Everything You Need To Know About Twitter Replies and Mentions:

Mentions were adopted by Twitter when people started using @replies to reference other Tweeps in their tweets. The difference between replies and mentions is that when you mention someone you might not be talking directly to them. In this case, all of your followers would see your Tweet regardless of whether they are following the person you are mentioning or not.

Mentions won’t have the “in reply to ..” text because they are not necessarily directed at anyone.

Also, IIRC, Tweets starting with an @username won't show for your followers, regardless of whether they're replies or just mentions.