Journal articles contain very specialised research which is often never looked at for decades. If you are studying say religion there could be am article dating from 50 years ago about a local organisation that is still valid even if some of the arguments have dated. In Irish history a huge amount of work on the period 1870-1890 was done in the 1960s and 1970s and there has been very little change to the available sources so the old work still holds up pretty well. It is also important to read conventional arguments for yourself and not just use other people's characterisation of them so reading the original is essential. [Edited to make the example more relevant]