ADC, DAC, and 2.95uA timers.

Can your Rasp. Pi Zero sleep at micro-amp currents, and self-wakeup 5 hours from now to perform some task, and go back to sleep? The specs here for this Arduino are downright ridiculous for low-power applications. We're talking about using 3x AA batteries for MONTHS. (3x NiMH AA will be 3.6V with 2000 mA-hrs. Using 3uA is 600,000 hours of battery, or 25,000 days of charge. You'll "only" last months because NiMH AAs will self-discharge before your Arduino actually uses all the charge).

And the Arduino here has all of the analog circuitry you need. 12-bit DAC means you have 4096-levels of output... a 3.3V signal can be varied in 0.8-millivolt increments. That's 0.0008Vs per increment.

The 16-bit ADC means that the Arduino can sense 0.05mV differences (probably at very low frequencies, but its an option with the right tradeoffs). Without any supporting chips. Alternatively, you have a 1MHz ADC (at some unspecified lower resolution. Probably 1MHz best-case 1-bit ADC), which means you can perform a million measurements in a second, to measure "rate of change" of your sensor.

The "1MHz" spec is useful for motors and other higher-speed equipment. If a Motor is rotating at 12,000 RPM, 10kHz ADC (1/100th of the max rate 1MHz, or 7-bits of precision per sample) will still be 500 samples per rotation, more than enough to get a good estimate on the motor's velocity.

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Application: 3xAA batteries, remote sensor that sends a WiFi packet every hour for temperature (or any other sensor that can be represented by voltage between 0V to 3.3Vs. Or even a sensor that needs to "sample" over a few milliseconds to get a value, such as determining the speed of a motor)

Stick a cheap 5V solar panel + some simple charge circuitry to trickle-charge the AA NiMH batteries, and you probably can run the rig indefinitely. (Really, you just need to ensure that the AA batteries don't leak current through the solar panel. So maybe a 0.7V diode to prevent backwards current, and you're set. 5V solar panel minus 0.7V diode outputs 4.3V nominal, but then becomes current-limited (probaly??) as it drops down to the 3.6V the batteries are set at. As long as you're only pushing milliamps, you're in NiMH trickle-charge mode which NiMHs are really, really good at).

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EDIT: I didn't even notice the integrated Li-Po charger. NiMHs are cool because they're bloody simple to use in trickle-charge mode. But if you've got a Li-Po charger built into the damn thing, you might as well use Lithium-Polymer cells instead.

There's a ton of complexity involved in safely charging, and discharging Lithium-Polymer batteries. But hell, if its built into the Arduino, might as well use it. In this circumstance, the 5V solar panel is plugged into the Arduino somehow, and the Arduino charge-circuitry will then safely handle the decisions involved in charging / discharging the Li-Po battery (also hooked up to the Arduino).

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EDIT2: I guess a "normal" Arduino can do this at low power consumption too. The reason to get "Arduino Pro" is for integrated Wifi + 480MHz processor. So you'd want some "difficult calculation" associated with the sensor reading... maybe a local GUI with a screen + Javascript to simplify the GUI-design.

The Arduino can sleep most of the time, but if a user comes up to maintain the device, they can click a button, bring the Arduino out of sleep mode. It boots up to a Javascript web-GUI for maintenance, and then sleeps back down to 3uA sleep mode once the user is done with local maintenance.

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100 Mbit Ethernet can have PoE, so any Ethernet Wire could transmit power instead (to a Rasp. Pi Zero). The Arduino's niche would be some kind of wireless, remote-power situation that requires very, very low power consumption on the scale of weeks.