No problem
at least you can understand what I say -- unlike others who can't seems to be able to understand simple english.
AFAIK, this practice is for safety reasons: some SLG MLs will check the databse using your whatsapp number to see if you have patronize other MLs, and to ensure you are a "well-behaving" customer before responding to your whatsapp. (just like we read the forum before deciding which ML is worth a visit). I think SLG or someone even build a wechat interface so that the ML just need to text your number to a robot account for checking. Some ML won't respond to numbers which do not appear in the database -- but I know Vera is more open to accepting new customers that do not have records.