The word purpose means a reason for something existing or happening. A target means a goal. A target can be either physical like a dart board or more abstract like number of sales per year.

Target 目標 Purpose 目的

'Target' is generally more specific and is would tend to be used more in cases where the goal can be expressed as a quantity. "Our target is to double sales by the end of this year." "His campaign targets the middle class." [Note also that 'target' can be used both as a noun and a verb] A purpose is more general and more mental than physical (usually). "Our purpose is to provide a better life for this country." "My purpose is to educate people in better ecology." Also, a machine or device can have a purpose, but not a target (unless it's something like a missile.) "A car's purpose is basically to transport people." "This application's purpose is to re-order data in a clean way."