Ok, imagine this:
I am building a small extension at the front of the house, commonly called "a porch".
I have to quote you, so I give you a choice:
1. Full and final price all inclusive £8k.
2. Pay for material £2k + other expenses (diesel, running errands, tools wear and tear, insurance, etc.) £2k + day rate for 2 people £350 × whatever it takes (might be 2 weeks, 3 weeks, 4, 5, 6 weeks, etc.). This depends on weather because if it rains hard and we can't work, you still pay us to be there.
If material is not delivered on time and we're stuck waiting, you pay us.
Every time we feel like having a break, you pay us.
Every time some neighbours have a chit chat, you pay us.
Plaster drying? You pay us while we polish every brick outside and fix that micro imperfection on the pointing.
Silicone has run out...Let me go to the hardware store and get some, in the mean time guess what??? Yes, you're paying us.
Now, does the fix £8k quote sound more appealing?