It's pretty easy to create an extreme example where a higher overall return is handicapped by higher volatility.
Imagine there's an investment called CrazyStocks. CrazyStocks averages 15% per year returns over 50 year periods. Over this 50 years you can guarantee it will drop 95% at least 2 times, but it will slowly climb back up to eventually average out to 15%/year after 50 years.
Now, if you could just leave the money alone for 50 years, say during accumulation, obviously 15%/year is great, so there's no reason not to do it. The problem comes when that 95% drop happens, and you're relying on that money to eat/pay doctor's bills/pay rent/etc. Now you're screwed, because you just spent 50-60% of your portfolio in a down year or two, and you aren't able to take full advantage of that recovery.
This same phenomenon happens on a smaller scale when talking about regular stocks. In general crashes happen fast, and recoveries happen slow. If you are living off pure stock returns, you will eventually have to sell them during down years, sometimes significantly down. Having bonds in your portfolio allows you to use the bond money (effectively rebalancing) during those down times, and lets you insulate yourself some from the downturn. During the accumulation phase you're never spending the money, so having less/no bonds isn't as risky.
Obviously we can't predict the future and know the exact sequence of returns OP will be dealing with, but in general most of what I've read found 10-20% bonds worked out better in the long run when people were withdrawing from an investment on a regular basis. Whether future bond/stock return rates will hold true to the past is up for debate, but we need to keep in mind that overall long term return doesn't tell the whole story when we're withdrawing from an investment.