Example:
3 minutes into the game, both players have established their basic economy of Minerals and Vespene. Worker scouts were sent and returning (if lucky), when one of the players (Zerg) decide to occupy a Converter. Instead of gas milking, the player is massing infantry for a strong push (Matter Conversion at main base).
With the extra income, the Zerg player is able to support more Hatcheries, Overlords, Drones and Zerglings (with occasional Hydralisks when the cooldown on Matter Conversion initiates). He has a natural and 2 additional Hatcheries in main base, but he has yet to access tier 2 tech.
Skipping a bit ahead, the Terran player has successfully (but suffered casualties) fended off waves after waves of tier 1 Zergling and Hydralisk swarms, by MULEing, walling and teching to Hellions. He has access to Banshees and Cloak, and to Reapers (harass has trapped him for quite long enough). A secret expansion has been set up.
By this time, the Zerg player has shortly accessed Lair tech and is in need of vespene for higher tech units (an expansion with 2 operational geysers counts for 1 geyser in SC1 terms, since the amount of vespene has been reduced, more workers needed, etc etc). He switches Matter Conversion to his expansion to maintain his expansion's integrity from reaper and banshee harassment, whilst safely draining his main base geyser of its Vespene.