In R, the plm function documentation of package plm, we can read about possibility of choose one of three effects individual, time, twoways. Why such exists if I can just pick model type which already specifies which effect to use ? E.g. 'within' model will only use individual and random will always pick twoways. To say more - for example pooling model by definition takes no effect (no time and no individual) so choosing effect in this case is meaningless. What's the purpose of this additional input?
How do you come to this conclusion? The within model can be used with "individual", "time" or "twoways". You should see different results for your model coefficients, when choosing a different effect. Also, for example, when you use "time" or "twoways", you should be able to get the specific time effects via
summary(fixef(yourmodel,type = "level", effect="time")).
(My plm package version is 2.2-4.)