You will always need a way to know from where you got the lead.

• Go to Setup -> Leads -> Sources and add new source depending on your needs.
• To setup default lead source navigate to Setup->Settings->Leads and chose Default Source.

You can assign a source per lead and you will know from where your leads coming and from where you have the most leads converted to clients when generating report.

The default lead source is used to auto populate the source field when creating new lead, each time you add new lead this source will be auto selected.