Skype works best for video chat, or voice chat, although you can also use it for text (typed) chat. For language practise, video does work well, as you can see the facial expressions of the person you are talking to, which helps to know if they are joking, and also can help with things like explaining how to use your tongue to make certain sounds. italki messaging is pretty awful! it works, but only just. I have lost several messages when I am looking at different pages. Maybe they have got it fixed more recently... There are several alternatives to skype. Chinese people use qq or wechat, there are several others. It's convenient to use what the other person is already familiar with. It is also good to have a couple of options - sometimes skype has a bad connection when other programs are still OK.

I am amazed at how much better a video conversation is than voice alone. When you are learning, either with a teacher or a language partner, it helps to be able to see expression... and to be able to make gestures with your hands. Skype has a number of helpful features. I usually have the message (chat) window open on the side. Often I will type a link, to an image perhaps, and my language partner can click on it, and vice versa. You can tell Skype to show your partner your computer screen, instead of you, so you can show your partner anything you can bring up on your screen. It has nothing to do with Skype, but one of my instructors likes to use Google Docs at the same time as Skype, and that is extremely useful--I can type into Google Docs and she can underline errors or make corrections, etc.

Skype provides instant messaging and video chat, so you can have real-time conversations and practice speaking and listening as well.

Ah i wonder about this too. I understand the benefits of Skype but I don't have the time to have a video chat so have to stick with messages which suits me here on italki. I want to greatly improve my Japanese but for now just learning more is good and I vocalise the messages I get.

...or are you wondering why people are so eager to use Skype? (When, as you've said, the messaging function works quite well.)