order of elliptic curve divisible by prime Why order "u" of an elliptic curve "E" defined over a finite field "GF (q)" (u = | E / GF (q) |) must be divisible by a large prime number r to be appropriate for cryptographic purposes?
You want to work in a large subgroup of prime order because you want that for the ECDLP in the subgroup you are working in the generic bounds for solving the ECDLP (only square root attacks) apply and thus the security parameter can be chosen accordingly. 

If you would work in the entire group the ECDLP is attackable by algorithms such as Pohlig-Hellman, Pollards' $\rho$ method or the babystep-giantstep algorithm. Additionally one avoids curves (such as supersingular ones) that are susceptible to the MOV and SSSA attack (such that you can even use subexponential algorithms).