Medics care about these clots because fragments can embolise (break off) into the circulation. The path they travel narrows at the lung (pulmonary) arteries, where these clots get stuck and reduce or stop blood flow beyond the blockage (pulmonary embolism). This affects the lungs function and can kill. People feel breathless, and sharp chest pain when breathing in (pleuritic pain). If a patient with a swollen leg has chest pain or breathlessness - consider PE.

Note the mimics of DVT:

A few diagnosis can look similar to DVT’s, including a Bakers cyst, varicose eczema, thrombophlebitis, and cellulitis. The treatments are different so being able to distinguish between them is important. History, examination, and investigations can exclude or confirm these causes. A DVT can often be caught on an ultrasound scan.