Taken as a daily dose of 2.5mg or 5mg, it helps the nerve bundles to repair post-op - which hopefully increases the chance of recovering erectile function. The 2.5mg / 5mg dose is also used for enlarged prostate. For some men, the 5mg daily dose is enough to be able to get erections on demand (but not usually in the first few months after the RP) while others will have the 10mg or 20mg dose just for events in the same way that you use sildenafil.
Until fairly recently, almost all CCGs approved daily dose tadalafil post-RP but now there are many areas where approval has been withdrawn and men really have to fight to get it prescribed. The prescribing guidelines are supposed to be:
2.5mg / 5mg tadalafil daily for BPH or as a prophylactic post-RP OR
4 x 20mg tadalafil per month OR
4 x 50mg (or 25mg / 100mg) sildenafil per month to have sex (although NICE has recently said that men can have it daily if they can convince their GP that they need to have sex every day, many CCGs are still not allowing this) OR
4 x 10mg / 20mg levitra per month OR
4 x caverject / muse / invicorp per month
I find it shocking that, having had cancer and cancer treatment that leaves him unable to get an erection, a man is rationed by NICE to an attempt at sex once per week. I think it is absolutely scandalous that some CCGs and individual GPs can then decide to not even prescribe the minimum.