Until and unless you don't extract the plant powder in a solvent, how will you test the presence of a particular phytochemical. You will need to extract the plant powder for solubilizing the constituents according to their solubility and affinity. This will not only help you to test them but also help to simplify the isolation process.
Direct plant powder can be used for anything only after knowing it's composition. This can only be found be chemical testing and for that you have to follow the above method of extraction using a solvent.
Extraction is recommended as it facilitates the extraction of particular type/group of phytochemicals of your interest.Powder can be used only for certain tests.Which tests do you want to carry out?
plants are composed of complex chemical constittuents which are broadly
1.chemicals synthesized for their own consumption and use like chlrorophills, protiens, sugars, etc
2- primary metabolites,
3. secondary metabolites.
for human theraputics the secondary metabolites are considered will be in minor concentrations ex not more than 1or 2% of total weight of plants. so for better theraputical efficiency we need to reduce the bulk plant powder amount administration. so we do extraction with various solvents and methods to reduce the quantity of bulk of powder and with less amount of extract with high concentration of phyto constitutens for efficient action. this will further help in pin pointing the chemical responsible for the biochemical action and c onfirmation, isolation.