The Western blot technique has many steps, and often the whole procedure may require several hours before result can be recorded. 

An enzyme, like alkaline phosphatase (ALP) or horse radish peroxidase (HRP), or a fluorescent dye, like DyLight®, attached directly to a primary antibody allows shortening of the protocol by many incubation steps. Incubation with a secondary antibody and washes are to be omitted when directly conjugated primary antibodies are used. 

Primary antibodies can be easily conjugated, as available kits are very well optimized to be time and labelling efficient. This approach is recommended for detection of proteins of high and moderate expression. 

Agrisera's collection of directly conjugated primary antibodies to various plant and algal proteins can be found here

 How to save time in Western blot?