Why does Sherlock only send Watson to the moorlands?

Sir Henry: 'Is it possible that you could come yourself, Mr Holmes?'

Holmes: 'If matters came to a crisis I should endeavour to be present in person; but you can understand that, with my extensive consulting practice and with the constant appeals which reach me from many quarters, it is impossible for me to be absent from London for an indefinite time. At the present instant one of the most revered names in England is being besmirched by a blackmailer, and only I can stop a disastrous scandal. You will see how impossible it is for me to go to Dartmoor.'

Sir Henry: 'Whom would you recommend, then?'

_____Holmes laid his hand upon my arm.

Holmes: 'If my friend would undertake it there is no man who is better worth having at your side when you are in a tight place. No one can say so more confidently than I.'

It is possible that Holmes' planned to fool his adversary into complaisance then secretly travel to Dartmoor on his own, but this is only speculation.

By the way, unless Watson made up his appearance or changed the details in his telling of the case, this blackmailer could not be Charles Augustus Milverton.