They are two different but incomplete parts of a fuller sentence.
- When the shop is open ---> I will do x.
- When the shop is open --->I will buy some eggs.
- When the shop is open ---> we can get some chocolates for the children.
- Etc
- When will the shop be open --->I need to buy some x.
- When will the shop be open ---->I need to buy some eggs.
- When will the shop be open ---> We have to buy some chocolates for the children's birthday party.
- Etc
First sentence is used for what will happen when the shop opens.
Second sentence is usually used when you are asking a question or explaining that you need the shop to be open for a reason.
A When will the shop be open?
B It opens at 9:00 am.