Doesn't the specific pack that I buy become an isolate as soon as I bring it home and use it?
No, the culture becomes the offspring of an isolate. An isolate is the result of plating a culture for single colonies, and then selecting a colony (a.k.a. colony-forming unit) for transfer to slant or propagation. Plating for singles will often produce cultures with slightly different performance characteristics due to the fact that strains will often produces cells with slight genomic and/or phenomic variations over time. For example, W-34/70 and W-34/78 are isolates from strain number 34.
If one examines the plate in the photo shown below closely, one will see a few well-isolated near-perfectly round colonies in the lower right-hand quadrant. These colonies are all the offspring of a single yeast cell; hence, they are isolates.
Often, multiple colony-forming units (CFUs) will be selected for transfer to slant. The slants will then be used to propagate cultures, which, in turn, are used to test fermentation performance. The isolate that performs best is selected for use in real-world brewing. The other isolates are held in reserve or discarded.
Let me give you a real-world use of isolation when working with a single-strain culture. Let's say that we like a yeast strain's flavor, but we want to see if we can alter how it flocculates. We do so by cropping early, plating for singles, testing the isolates, and brewing with the most favorable isolate. We can repeat this process several times to see if the flocculation performance improves. Conversely, we can push flocculation in the other direction by cropping late and performing the same isolation process.