We’ve been here at few times before. I don’t like the term resonant antenna (it’s misleading). Some [on this website] have said their antennas are resonant across the band because they claim 1:2 VSWR. If you’re running QRP, you can’t afford to throw away that much of your precious RF power.
I tune my dipoles [and linked dipoles] for the small range of frequencies I tend to operate in [CW sub-band]. The 30m band is narrow enough to achieve a good VSWR at both ends of the band. But if you like to operate CW and SSB on 80m, 40m, 20m 10m, etc better use an ATU. Hence the advantage of those CW QRP rigs that have a built-in ATU.
73 Andy