Pre-heating your diesel fuel dissolves paraffin wax crystals (and ice) that form when diesel fuel is chilled.

Pre-heating your diesel fuel enables filters and water separators to work more efficiently.

Pre-heating your diesel fuel prevents fuel filters from plugging with wax and/or ice crystals.

Examples:

ATG Diesel-Therm - Fuel Preheater for Diesel Engines

Racor Diesel Fuel Heaters

Arctic Fox Hotline Electric In-Line Diesel Fuel Heater

Universal Diesel Fuel Filter Preheater - YouTube