Consent is not only physical and sexual, it's all encompassing. It's intrusion of someone's private space without prior permission.
If you call someone whom you don't know well without asking if they are free or interested in taking your call, it's breach of consent. If I write you love letters when you have expressed you don't want to receive them, it's breach of consent. This is the prime reason why I don't respond to unsolicited WhatsApp texts, because WhatsApp is a private space for me (I highlight this in my WhatsApp about) and one should rightfully ask for my number over email or FB and if I am eager to share, it means I have given permission to reach me there.
Many times, one might be interested in engaging but still if you take permission before intruding their private space, it displays that you understand the meaning of the word no - that even while you two are in the middle of the act (could be sex, conversation or even a call) and the other person mentions that s/he doesn't want to continue any further, you get it and do not bother them again.
Consent is the wisdom that just because it is interesting/important/urgent for me doesn't mean it will be interesting for others.
Gangs of Wasseypur in that iconic scene where Huma scolds Nawazuddin when he touches her hand without permission (even though she doesn't mind) is a great tutorial of how consent works. Your asking helps the other person know that you understand no - and no is no irrespective of when it is said.