When I run a JC-B, there are some items that show zero for an estimated & projected cost.  
 
These items are type "R" and have last & avg costs.  The item is in inventory and we did not purchase it for the WO.

What if we don't use standard cost?  
 
Also, why do some item have a standard cost & others don't?

Even if you don't want to base any actual transactions on standard cost, it is still a good idea as John says to populate the fields and do the cost rollup in BM-G to get assemply standard costs.   The standard is simply a benchmark of what the item "should" cost and is used as the Estimated cost in Work Orders and Job Cost reports and is also used by the Estimating module.  
 
If your component prices are volatile, you may run IN-L-A and BM-G frequently to keep the benchmark current but if you are in a more stable market, then quarterly or even less frequently is fine.

Unless you enter and roll up the standard cost, in IN-L-E, IN-L-A, IN-B or BM-G, there will not be a standard