In the motivation letter you could address the following questions:

• What is the added value of participating at the conference for your research project or your carreer?
• Will the contents of your conference presentation get published (e.g. conference book or report)?
• How important is your participation at the conference for your research group and how will you communicate the content of the conference to other members in your group?
• Is there any special event included in the conference participation (e.g. special meeting of young researchers, satellite meetings, special workshops)?