I thought it was pretty clear what the advantage was. If you need it here's a super simple explanation:

• higher engine compression better than lower engine compression
• FIA rule says all engines must be the same fixed compression size
• FIA also says engine size measured when engines cold
• Certain materials change size with temperature
• Some engine teams figured out that they can make their engine produce slightly higher compression at high race temperatures
• Result: some engines have higher compression than others yet all engines legally pass the fixed compression size rule.