Here are just a couple of examples
Create a circular pattern of sketch centerpoints for a bolt hole circle and then exit sketch and place the holes. Now go back and edit the sketch to increase or decrease the number of sketch points. Rebuild. The number of holes does not change.
Create a pie piece shaped sketch. Extrude and then circular pattern the feature.
Do the same thing except change your radial lines for the pie piece to construction lines and pattern the arcs in the sketch.
If your lucky you won't get a redundant coincident points error when you try to extrude the sketch. Now try this with experiment with something significant - like a gear or sprocket tooth profile. Have fun.
There are times when a sketch pattern make sense but whenever a feature pattern will create the same geometry opt for the feature pattern. The simple sketch will be much more robust than the complex sketch pattern and the pattern will be much easier to edit.
When you create a sketch pattern think of all of the constraints that Inventor must keep track of. For example imagine the profile of the gear tooth or sprocket tooth suggested earlier. Normally I test a sketch by trying to drag endpoints. All of the relationships of those sketch entities in a patterned sketch must be calculated by the computer.
I have seen many many other examples. Maybe someone else has some good ones.
Again, I did NOT say never use sketch patterns. If I don't emphasis that I'm sure someone will want to interpret that I did.
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