Both have their advantages and disadvantages. When you book your entire holiday in advance including flights, hotels, transfers, activities, it gives you the peace of mind that you will be able to experience the holiday the way you had planned. There will no availability issues. But to plan your holiday and book all services in advance, you need to research the destination on your own or take advice of your travel planner or travel agent and finalize your plan.
A lot of people get thrill in planning only the necessary services like air tickets, hotels etc. and remaining they like to reach the destination and then decide what they prefer to do. This gives you lot more flexibility to do things that you like to do.
When I went to UK with my wife, we did not book any activity. We booked the flight ticket and hotel close to the tube station (similar to metro) and booked our airport pickup. We also wanted to watch the football match between Arsenal and Manchester United live in a stadium in #Manchester. So we booked these tickets in advance. I had been to London before so knew about the destination. We finally did just a city tour and preferred exploring the streets, eating food in local joints and exploring the local culture.
We were comfortable pre booking only the important services because we were aware about the destination. I have done trips where everything was booked in advance for e.g. Dubai, #Pench national part etc.
To conclude, I would recommend that if you are an explorer and if you are comfortable with a destination based on your prior travel experience or have the confidence to figure out the unknown, go with flexibility. If you are starting out or prefer everything planned properly to avoid surprises, I suggest you book the entire holiday with #sightseeing in advance.