Hello,
There is a lot of discussion regarding these two issues. Some of the more seasoned members will know how to link up previous discussions for you to learn more about this subject that you never knew you always wanted to know. But here is my 2 cents.
I have used the same Packa for the past 3 Caminos. On the first one, I used it twice. On the second one, I used it 4 or 5 five times. Early this year I probably used it for at least 15 days. It rained like crazy. On all 3 of my Caminos, I was there at the same time of year. So figuring out the weather is out of my ones control.
The Packa, or a Parka (Many styles around) covers the back pack and keeps it dry. If you wear a rain coat and use a pack cover during heavy rains then the shoulder straps get wet and eventually the pack will too. I bring a rain coat too. I use it more as a wind breaker but during light rains it is sufficient with a pack cover. By the way, the Packa is expensive but after 3 Caminos, I have got use of its value and cost. Most of the folks purchase an
Altus in France or Spain. There are at least half the cost of the Packa. What you don't want is one of those cheap plastic ones that cost a couple of dollars. They won't hold up in the rain.
So now I must warn you. Please be prepared for a lot of discussion. This is one of the popular subjects. After this one, you can ask what kind of footwear to bring along with you. That will keep you busy until you leave for Spain.
Buen Camino