The yeast included with a kit is likely to be the smallest amount they thought would work, not the amount to produce the best beer. In addition, you don't know how it was stored. If you go to your LHBS, you'll find yeast that has been looked after. The ones you buy separately are larger so that you get a good does of yeast in to your wort. Under pitching (putting in too small an amount of yeast) can result in a less desirable flavour profile. Though just to prove that most rules have exceptions, there are certain beer styles (some of the Belgians) where brewers will deliberately under pitch (compared to the usual notion of how much is best) to encourage the formation of certain flavours associated with the style.
Using US-05 or S-04 will be great for most straightforward kit ales. As you get more experienced, you will probably want to explore more of the wide palette of styles out there, and then there are a few more dry yeast options (T-58 is a favourite of mine), but the liquid yeast strains offer a much greater range.
2c,
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