The more you know about your customers, the easier it is to speak their language.  When you do that, they are more likely to respond positively to you, your product, and your company.

Even if you have defined your ideal client that may be too broad a group to speak to all at once.  Personas define individual people who clearly represent a subset of your ideal customer group. This allows you focus your product designs and marketing messages for specific people instead of some anonymous group.