Well the MS will provide you with a molecular ion and a fragmentation pattern, then you can make an educated guess as to the structure, especially if you know the starting materials and reagents.
The LC will allow you to run the reaction and follow it on a time axis so you can see the build up of products and consumption of starting materials. Also if you use a calibrated LC (quantitative) you can work out some reaction kinetics.
Remember to run your starting materials through the system to collect all the data.