add a new item to the UITable view vs. reloadData - ios

Question

the doc about reloadData of UITableView says(http://developer.apple.com/library/ios/#documentation/uikit/reference/UITableView_Class/Reference/Reference.html): "it should not be called in the methods that insert or delete rows, especially within an animation block implemented with calls to beginUpdates and endUpdates".
My question is, why? (especially first part, italic).
There's a book which I am reading which implements adding items to the UITableView like this:
// Add new item to the table
- (IBAction)addNewItem:(id)sender
{
// Update the model, add a new item
BNRItem *newItem = [[BNRItemStore sharedStore] createItem];
// Figure out where that item is in the array
int lastRow = [[[BNRItemStore sharedStore] allItems] indexOfObject:newItem];
// Create the corresponding index path
NSIndexPath *ip = [NSIndexPath indexPathForRow:lastRow inSection:0];
// Insert this new row into the table
[[self tableView] insertRowsAtIndexPaths:[NSArray arrayWithObject:ip]
withRowAnimation:UITableViewRowAnimationTop];
}
whereas same could be achieved also like this:
// Add new item to the table
- (IBAction)addNewItem:(id)sender
{
// Update the model, add a new item
[[BNRItemStore sharedStore] createItem];
[[self tableView] reloadData];
}

Answer 1

By reload data you're forcing all rows to be recreated so when you have information what is new it is just much more performant to tell table view what have to be added/removed, especially when, for example, you added one row at the end and that part is currently not on screen (so view does not have to be re-rendered, only scroll high have to be recalculated).