The client doesn't always wait for your free moment.
They write when they have time: at lunch, after work, in the evening, at the weekend. If they don't get a reply right then, they often look for an alternative.
Not because you're not good. Because the need is here now.
WhatsApp feels quick because each message takes a few seconds.
The problem is that a single message never arrives on its own. You get questions, confirmations, time changes, availability requests, clients writing after hours and conversations left halfway.
In the end you don't just lose time. You lose focus.
If you work without a secretary, every message is a small interruption.
One client asks about availability. Another wants to change the time. Someone writes in the evening, someone else while you already have a person in front of you.
Replying matters. But if you always do it yourself, your day gets broken up constantly.