I really didn't understand what "germline dna" truly stands for. Everyone is saying that germline mutations can be identified with blood cells while somatic ones can only be identified from the tumor tissue where somatic mutation occurred in. I don't understand how we can see germline mutations from peripheral blood cells.
Yes, as per ATPoint, it is matter of convenience and ease of availability.
From the blood, we can specifically isolate what are assumed to be healthy lymphocytes/leukocytes from the buffy coat layer after centrifugation of the blood, which in addition segregates the red blood cells from the blood plasma. These leukocytes, in a cancer setting, are assumed to have been produced relatively recently from the time of blood withdrawal and using the germline genetic code. People then compare these to the tumour to infer somatic mutations, as I was doing in my PhD in addition to circulating free DNA analyses: https://www.ncbi.nlm.nih.gov/pubmed/21990379