Ahsoka was necessary to promote her series.
She made sense as well appearing just given the timeline and 'where' her character is/was at as far as storyline.
I'm hopeful that with the less restrictive backstory and 'lore' that her character and the characters from Rebels that have been confirmed to be in the series and rumored to be in there that we see some good storytelling. I'm hopeful that with Filoni essentially writing this show we will get some of the best SW's content and I'm trying to curb my expectations so I won't be disappointed if the show doesn't live up to them. Rebels IMO was really well done and I hope that momentum continues with Ahsoka's series.
Fortunately I've watched scarce nits and pieces of the animated series, so I have no preconceptions or expectations from them.
In regards to the Ahsoka series....I'd think some sort of Youtube Recap of 'Rebels' would give you all the pertinent information you'd need to understand 'who' the characters are that are being introduced and 'why' they're doing what they're doing. Just from the characters confirmed.....Thrawn and Sabine.....and heavily rumored due to casting calls....Ezra.....it's a pretty safe bet that this show is more or less going to be a continuation of Rebels and where that story left off.
Although with 'only' 4 Seasons (75) episodes @ 22 minutes each....so, 28ish hours of content......I'd recommend hammering that series out. It's a fun one with little 'down' time.
Whereas Clone Wars has 7 seasons (133) episodes....of which, much of the early seasons are very hit or miss. The battle of Geonosis is pretty cool....and it really starts to find footing in S3. But it's SO much to take in.....but worth it IMO. It paints a true and full picture of Anakin and Obi Wan....way more than you can glean from the limited Sequel series. And of course it details Ahsoka's journey.
AND the last four episodes of that series "Old Friends Not Forgotten", "The Phantom Apprentice", "Shattered" and "Victory and Death" (whose intention was to be viewed as one 'film' so to speak) is IMO some of if not the best SW output to date. From the score to the story, the action....everything....it is incredible.