The yields for treasuries are very low, it is not logic to hold a treasury for 30 years to get two percent every year, I'm a beginner so I wonder if buy bonds/notes and waiting the yield to fall and its price to rise to sell the bond is their strategy?
If treasuries are at 2% then 2% is the risk-free rate of return. If you are a risk-adverse investor or if you want to allocate some percentage of your portfolio to low-risk assets either permanently or temporarily while you look for a better place to invest it, you'd want to hold some quantity of risk-free assets like treasury bonds.
If you're a 30 year old middle income American, you almost certainly shouldn't be 100% invested in long-term treasuries in your 401(k)-- you need to accept some risk for the potential to get much higher growth over the decades before you retire. On the other hand, it wouldn't be crazy to have 5% invested in safe bonds as part of a reasonable asset allocation strategy. Or to have that amount increase over time as you get to retirement and beyond. A 100 year old investor probably ought to have a lot more safe assets than a 30 year old.