Would be primarily utilized by risk-averse investors. The 60/40 portfolio has had a rough ride over the last while because of the pressure on bond pricings due to rapidly falling then rapidly escalating interest rates.
There is some bullish cases in regards to bonds right now because of falling interest rates, so ZBAL gets a bit interesting from that front as well. A portfolio of bonds and equities could certainly outperform a portfolio of pure equities over the short term if rates continue to fall.
Overall, great fund, should provide reasonable returns with lower volatility. Just don't expect it to return as much over the long-term as a pure equity fund or even a 80/20 like ZEQT